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ELEMENTARY  SCIENCE 


BY 

JOHN   G.   COULTER 


ILLUSTRATED 


CHARLES   SCRIBNER'S   SONS 

NEW  YORK          CHICAGO  BOSTON 


COPYRIGHT,  1917,  BY 
CHARLES  SCRIBNER'S  SONS 


PRINTED  AT 

THE   SCRIBNER  PRESS 

NEW  YORK,  U.  S.  A. 


FOREWORD 

Before  the  manuscript  of  this  book  had  been  sent  to  the 
publisher,  the  author  had  gone  to  France  in  the  American 
Ambulance  Service.  The  proof,  therefore,  has  not  had 
the  advantage  of  the  author's  revision.  The  undersigned 
was  familiar  enough  with  his  general  purpose,  however, 
to  undertake  the  final  reading  and  to  make  such  minor 
modifications  as  seemed  desirable.  The  field  covered  is 
so  broad  that  no  one  could  be  expected  to  have  a  critical 
knowledge  of  all  the  facts  presented,  but  they  have  been 
obtained  from  the  most  reliable  sources. 

It  is  to  be  expected  that  each  specialist  will  be  most 
critical  in  reference  to  his  own  subject,  but  the  chief  mo- 
tive of  the  book  is  to  develop  some  appreciation  of  nature 
as  a  great  synthesis  and  of  our  relation  to  it.  To  present 
so  general  a  picture  with  the  least  confusion  involves  the 
omission  of  many  details.  Perhaps  no  two  independent 
teachers  would  agree  exactly  upon  the  selection  of  ma- 
terial, but  this  is  of  minor  importance  as  compared  with 
the  purpose  in  view. 

The  colloquial  method  of  presentation,  accompanied 
by  more  or  less  repetition,  was  adopted  because  it  had 
proved  to  be  the  most  effective  as  tested  by  many  trials 
with  classes.  Even  here  it  is  recognized  that  much  de- 


IV  FOREWORD 

pends  upon  the  personality  of  the  teacher.  No  style  of 
presentation  is  probably  best  for  all.  It  is  hoped,  how- 
ever, that  the  point  of  view  developed  by  the  material 
selected,  and  presented  in  a  way  that  has  been  found  to 
be  effective,  may  be  helpful  to  teachers  who  wish  their 
pupils  to  realize  that  one  must  live  in  intelligent  partner- 
ship with  nature,  and  therefore  must  become  acquainted 
with  nature. 

JOHN  M.  COULTER. 


CONTENTS 

CHATTER  PACK 

I.    INTRODUCTION i 

II.    SOME  RELATIONS  BETWEEN  WATER  AND  LIGHT  8 

III.  THE  DISTRIBUTION  OF  WATER 16 

IV.  EFFECTS  OF  THE  MOVEMENTS  OF  WATER      .  24 
V.    SOLUTION 40 

VI.    MAN'S  SUPPLY  OF  WATER.     THE  INSIDE  OF 

THE  EARTH 46 

VII.    THE  SUCTION-PUMP.    ATMOSPHERIC  PRESSURE  55 

VIII.    ARCHIMEDES'   PRINCIPLE.     WATER-PRESSURE 

AND  WATER-WORKS 63 

DC.    WATER-POWER  AND  STEAM-POWER      ...  70 

X.    WATER  AND  AGRICULTURE 79 

XI.    ORIGIN  OF  SOIL 86 

XII.    KINDS  OF  SOIL 94 

XIII.  FERTILITY  AND  SOIL  LIFE:  BACTERIA       .     .  101 

XIV.  MECHANICAL  WORK:   SIMPLE  MACHINES  .     .  109 
XV.    WORK  AND  ENERGY:    GRAVITATION     .     .     .  117 

XVI.    INTRODUCTORY  AS  TO  HEAT:    DISCOVERY  OF 

FIRE 123 

XVII.    MEASUREMENTS  OF  HEAT 128 

XVIII.    HEAT  AND  COLD 135 

v 


vi  CONTENTS 

CHAPTER  'AGE 

XIX.    SOME  EFFECTS  OF  HEAT 141 

XX.    THE  ATMOSPHERE 147 

XXI.    AIR  AND  WATER  COMPARED.    ATOMS  AND  ELE- 
MENTS       155 

XXII.    HEAT  AND  THE  ATMOSPHERE 162 

XXIII.  THE  SEASONS  AND  THE  SOLAR  SYSTEM     .     .  169 

XXIV.  WINDS  AND  WEATHER 178 

XXV.    ECONOMIC    IMPORTANCE    OF    WINDS,    RAINS, 

CYCLONES,  CLOUDS,  TORNADOES       .     .     .  189 

XXVI.    HEATING  AND  VENTILATION 199 

XXVII.    COMBUSTION 209 

XXVIII.    LIGHT,  COLOR,  AND  SOUND 216 

XXLX.    SOME    EFFECTS    OF    LIGHT.     PHOTOGRAPHY. 

PHOTOSYNTHESIS 225 

XXX.    FOOD.    THE  NUTRITIVE  CYCLE      .     .     .     .  232 

XXXI.    PLANT  LIFE 242 

XXXII.    THE  STORY  OF  SEEDS 247 

XXXIII.  PLANT  GROUPS 257 

XXXIV.  RELATIONS    BETWEEN    PLANTS    AND    THEIR 

SURROUNDINGS 265 

XXXV.    ANIMALS.    VERTEBRATES 270 

XXXVI.     INSECTS 2?6 

285 


ELEMENTARY  SCIENCE 


ELEMENTARY  SCIENCE 


CHAPTER  I 
INTRODUCTION 

Air,  water,  soil,  heat,  light,  plants,  and  animals  —  these 
are  the  principal  things  that  make  up  what  we  call  nature. 
You  will  find  that  your  life  is  a  sort  of  partnership  with 
nature.  To  live  in  it  the  best  way,  you  need  to  under- 
stand this  partnership  as  well  as  you  can.  You  need  to 
know  how  to  do  your  part  in  it.  Life  is  the  most  interest- 
ing thing  in  the  world.  You  want  to  find  out  all  you  can 
about  it  so  that  you  can  make  your  own  life  as  happy 
and  successful  as  possible.  A  good  way  to  go  at  this  busi- 
ness of  finding  out  about  life  is  to  study  first  the  things 
that  are  necessary  to  all  kinds  of  life,  plants  as  well  as 
animals,  and  see  how  these  things  work  together  to  make 
our  own  lives  possible  and  pleasant.  That  is  what  we  shall 
do  in  this  book.  We  shall  study  the  seven  things  mentioned 
in  the  first  sentence.  We  shall  study  the  conditions  that 
are  necessary  for  our  own  lives,  and  this  will  help  us  a 
great  deal  in  finding  out  how  we  ought  to  live. 

For  thousands  of  years  men  have  been  finding  out  more 
and  more  about  the  world.  Each  year  new  knowledge  is 
added  to  the  old,  and  this  knowledge  of  nature  is  called 
natural  science.  You  have  read  about  cavemen  and  other 
ancient  people,  and  you  know  that  the  savage  men  of  long 
ago  had  a  hard  struggle  for  existence.  They  did  not  under- 


2  ELEMENTARY  SCIENCE 

stand  how  to  work  in  partnership  with  nature,  and  so 
nature  seemed  more  of  an  enemy  than  a  friend.  There 
was  much  hunger  in  those  days.  Famine,  wild  beasts, 
and  cold  weather  —  these  were  enemies  against  which 
man  hardly  knew  how  to  protect  himself.  He  lived  "from 
hand  to  mouth."  Only  the  strong  and  hardy  survived  in 
those  perilous  times.  But  since  then  men  have  made 
hundreds  of  discoveries  about  nature.  These  discoveries 
have  made  it  possible  to  live  much  more  safely  and  com- 
fortably, until  to-day  even  poor  people  have  more  com- 
forts than  had  the  kings  and  queens  of  old. 

Sometimes  this  long  process  of  making  life  more  com- 
fortable and  secure  is  called  "man's  conquest  of  nature." 
But  this  expression  suggests  the  idea  that  nature  is  an 
enemy  that  we  must  fight  and  try  to  conquer.  That  is 
only  a  part  truth.  Man's  so-called  "conquest  of  nature" 
has  been  rather  the  learning  of  nature's  secrets;  learning 
how  to  take  advantage  of  the  many  ways  in  which  she  will 
help  us  if  we  study  hard  enough  to  understand  her. 

Those  things  that  we  call  inventions  are  simply  new- 
found ways  of  developing  our  partnership  with  nature. 
An  inventor  designs  a  new  type  of  engine,  or  a  doctor  dis- 
covers a  new  way  to  cure  a  disease.  We  call  these  things 
men's  inventions,  but  let  us  remember  that  they  are  sim- 
ply discoveries  of  new  ways  of  controlling  nature.  Man 
discovers  how  to  arrange  what  we  call  inventions,  but  it 
is  nature  that  does  the  rest.  Nature  supplies  the  materials 
that  man  arranges,  and  also  the  forces  that  make  all  of  his 
inventions  work.  So  you  need  to  study  nature  in  order 
to  understand  machinery  as  well  as  to  understand  about 
the  woods,  the  fields,  and  the  weather.  Nature  is  at  work 
in  a  steam-engine  as  well  as  in  plants  and  animals.  Nearly 


INTRODUCTION  3 

all  the  machinery  in  the  world  is  run  by  power  that  came 
in  the  form  of  sunlight  long,  long  ago,  and  was  stored  up 
in  the  form  of  coal.  Nature,  you  see,  is  the  very  founda- 
tion of  man's  work,  either  in  city  or  in  country.  So,  if  we 
are  to  have  a  real  understanding  of  our  lives  we  must  un- 
derstand about  nature,  and  our  place  in  nature,  as  well  as 
about  our  fellow  men  and  our  relations  to  them. 

Let  us  begin  by  thinking  of  the  world  as  a  whole,  and 
try  to  form  a  picture  of  it  in  our  minds  somewhat  as  you 
learned  to  do  in  geography.  Then  we  will  consider  it 
more  carefully,  one  thing  at  a  time. 

There  are  three  parts  to  the  earth.  We  live  on  top  of 
two  of  these  parts,  and  nearly  at  the  bottom  of  the  third. 
These  three  parts  are  the  solid,  the  liquid,  and  the  gaseous 
parts.  Most  of  the  earth  is  solid  rock,  the  liquid  is  water, 
and  the  gaseous  part  we  call  the  atmosphere  or  air.  Per- 
haps you  have  heard  the  statement  that  we  live  at  the 
bottom  of  a  great  sea  of  air.  It  lies  over  the  whole  surface 
of  the  earth,  and  extends  more  than  a  hundred  miles  above 
it.  It  is  a  huge,  soft  cover.  It  flows  more  easily  than 
water.  When  air  moves  so  that  we  can  feel  it  we  call  it 
wind.  Though  it  is  invisible  it  has  weight.  It  is  much 
more  dense  and  heavy  at  the  bottom  than  it  is  at  the  top. 
Most  of  the  atmosphere,  by  weight,  is  within  four  miles  of 
the  surface  of  the  earth.  Above  that  it  is  very  light  and 
thin. 

Our  earth  is  very  old.  It  has  not  always  been  the  pleas- 
ant place  that  it  is  to-day.  There  was  a  time  when  we 
could  not  have  lived  here.  There  was  a  time  when  there 
were  no  plants.  Before  that  there  was  a  time  when  there 
was  no  soil.  Nearly  the  whole  earth  was  covered  with 
water.  The  only  land  was  bare,  brown  rock.  Not  one 


4  ELEMENTARY  SCIENCE 

green  plant  grew  upon  it  Not  a  bird  flew  over  it.  Not  a 
single  animal  lived  there.  Everywhere  it  was  very  still 
and  very  lonely.  But  in  that  ancient  time,  millions  of 
years  ago,  the  earth  was  slowly  changing.  Slowly,  but 
surely,  it  was  getting  to  be  more  and  more  like  what  it 
is  to-day,  and  the  changes  that  went  on  then  keep  going 
on  to-day. 

In  those  days  when  the  earth  was  young  the  sun  shone 
just  as  it  shines  to-day.  It  was  the  heat  of  the  sun  that 
kept  things  changing  then,  just  as  it  keeps  them  changing 
now.  The  sun  warmed  the  surface  of  the  oceans  and 
drew  water  up  into  the  air.  Clouds  formed.  It  warmed 
the  air  more  in  some  places  than  in  others,  and  the  air 
stirred  and  moved  about,  somewhat  as  water  stirs  and 
moves  about  when  it  is  heated.  Winds  blew  and  carried 
the  clouds  over  the  land.  There  the  clouds  were  cooled  and 
changed  to  rain.  The  rain  fell,  and  the  water  ran  over  the 
land,  wherever  it  could  go.  The  air  and  the  water  and 
the  heat  worked  together  and  changed  the  surface  of  the 
rock  to  soil.  The  streams  carried  this  soil  down  toward  the 
sea.  They  made  it  into  mud,  and  banks  of  mud  were 
formed  along  the  shores.  Then  some  of  the  plants  that 
had  been  living  in  the  water  began  to  live  on  this  mud.  As 
more  soil  was  formed  these  plants  began  to  spread,  until 
at  last  the  land  was  covered  with  green.  These  green 
plants  made  food,  and  then  the  land  was  ready  at  last  for 
animals  to  live  on  it. 

Now,  with  these  things  in  mind  you  can  form  a  picture 
of  how  the  earth  changed  in  ancient  times,  and  of  how  it 
keeps  changing  to-day.  Light  and  heat  come  down  to 
us  from  the  sun.  The  air  is  heated  more  in  some  places 
than  in  others,  and  this  causes  movements  of  the  air. 


INTRODUCTION  5 

The  clouds  move  with  the  air.  Heat  draws  water  up  into 
the  air,  by  means  of  the  process  we  call  evaporation.  Light, 
shining  on  the  green  plants  that  cover  the  land,  enables 
them  to  make  the  food  on  which  animals  live.  Water, 
moving  in  clouds  and  falling  as  rain,  enables  the  plants  to 
live  in  the  soil  in  which  they  are  rooted.  From  the  soil 
and  from  the  air  they  get  the  substances  that  they  make 
into  food.  Here  we  see  one  of  the  great  partnerships  of 
nature.  Heat  and  light  and  air  and  water  and  soil  and 
green  plants  are  the  members  of  this  partnership,  and  the 
food  that  sustains  our  lives  is  one  of  the  results  of  their 
work  together. 

Of  the  number  of  things  absolutely  necessary  to  life, 
food  is  usually  the  most  difficult  to  obtain.  Air  to  breathe, 
water  to  drink,  and  enough  heat  to  keep  life  going  can 
usually  be.  obtained  by  all  living  things  without  much 
work.  But  with  food  it  is  different.  Nature  requires  that 
all  living  things  work  for  their  food.  Though  now  you 
may  be  getting  your  food  for  nothing,  somebody  else  has 
certainly  had  to  work  to  provide  it  for  you.  Of  the  money 
that  we  spend  for  really  necessary  things,  more  is  spent 
for  food  than  for  anything  else.  It  takes  a  great  deal  of 
work  to  cultivate  the  plants  and  to  raise  the  animals  that 
are  the  sources  of  human  food.  If  we  do  not  do  food- 
producing  work  ourselves,  we  must  pay  others  to  do  it 
for  us.  So,  as  we  study  the  working  together  of  the 
forces  of  nature,  it  will  be  interesting  to  see  how  these 
things  are  related  to  the  production  of  food. 

Let  us  go  over  this  matter  once  more  to  get  the  picture 
dear.  Think  of  the  heat  of  the  sun  that  starts  the  pro- 
cession of  changes;  it  starts  them  by  causing  movements 
of  both  air  and  water.  Think  then  of  the  water  falling  as 


6  ELEMENTARY  SCIENCE 

rain,  and  working  down  into  the  soil  as  well  as  running  off 
in  streams.  Then  think  of  the  plants  whose  roots  go 
down  into  the  soil,  and  whose  leaves  are  lifted  by  the 
stems  up  into  the  light  and  air.  From  air  and  earth  these 
plants  draw  materials  that  are  used  in  food  manufacture. 
These  materials  come  together  in  the  green  leaves,  and 
there,  with  the  aid  of  the  sunlight,  the  food  of  the  world 
is  made.  So  we  see  how  sun,  rain,  and  wind,  soil  and 
roots,  and  stems  and  leaves  all  help  make  the  world  a 
place  where  we  can  live.  They  are  all  necessary  to  our 
lives. 

Men  have  greatly  improved  the  plants  which  they  use 
for  food.  The  trees  in  the  orchards,  and  the  wheat  and 
com  in  the  fields  are  very  different  from  the  wild  plants 
that  were  their  ancestors.  Long  before  the  written  his- 
tory of  man  began,  our  primitive  forefathers  were  learn- 
ing through  experience  how  to  grow  plants  for  food.  Many 
famines  in  winter  taught  them  how  important  it  is  to  har- 
vest crops  and  store  them.  They  found  that  it  was  a 
good  thing  to  grow  certain  plants  in  places  where  they 
scratched  the  soil  with  sticks,  which  was  the  first  kind  of 
ploughing.  They  learned  that  the  food  plants  grew  better 
if  other  plants  were  kept  from  growing  near  them.  So 
they  weeded  their  little  cultivated  patches. 

So  you  see  that  ages  ago  man  began  to  work  as  a  partner 
of  nature  in  improving  the  conditions  of  life.  And  we  are 
still  engaged  in  that  great  work.  We  have  by  no  means 
solved  the  problem.  Each  year  we  are  finding  out  new 
things  about  nature  and  about  human  life.  So  each  year 
the  world  becomes  a  more  wonderful  and  interesting  place 
in  which  to  live.  Right  now  you  are  living  in  the  most 
interesting  period  in  the  world's  history,  and  to-morrow 


INTRODUCTION  7 

will  be  even  more  interesting  than  to-day.  You  have  an 
opportunity  that  no  one  has  had  before.  You  should  have 
a  great  deal  of  confidence  in  the  future,  and  be  determined 
to  make  the  very  most  of  this  one  life  that  you  have  to 
live.  Nature  is  sure  to  reward  you  very  richly  if  you  will 
obey  her  laws,  and  her  first  law  is  the  law  of  work. 

QUESTIONS 

1.  What  is  "nature"? 

2.  What  is  "natural  science"? 

3.  Is  nature  an  enemy  or  a  friend  to  man? 

4.  What  has  nature  to  do  with  inventions? 

5.  What  are  the  three  great  regions  of  the  earth? 

6.  How  is  soil  formed  ? 

7.  How  was  the  first  soil  formed  on  the  earth? 
3.  Which  lived  first,  plants  or  animals  ? 


CHAPTER  n 
SOME  RELATIONS  BETWEEN  WATER  AND    LIFE 

You  have  seen  that  in  order  to  understand  the  great 
"cycle  of  nature"  that  permits  us  to  live,  you  must  study 
the  following  things:  air,  water,  heat,  light,  soil,  plants,  and 
animals.  You  must  study  especially  the  ways  in  which 
these  things  work  together  in  producing  the  conditions  that 
permit  us  to  live. 

We  might  begin  with  the  study  of  any  one  of  these 
topics,  and  as  we  studied  it  we  should  find  ourselves  study- 
ing all  the  others,  so  closely  are  they  related.  We  will 
begin  with  water,  because  water  is  the  simplest. 

Water. —  As  you  look  at  water  in  a  glass  it  seems  a 
very  simple  thing,  easy  to  understand.  But  when  you  be- 
gin to  study  the  way  that  water  behaves,  and  the  many 
effects  it  has  upon  our  lives,  you  soon  find  that  this  is  a 
very  large  subject  indeed.  Three-fourths  of  the  surface 
of  the  earth  is  covered  with  water,  and  more  than  three- 
fourths  of  the  bodies  of  all  living  things  are  composed  of 
water.  This  suggests  the  idea  that  life  began  in  the  water, 
and  slowly  spread  from  there  over  the  land  and  into  the 
air.  There  are  many  land  animals,  including  man,  in 
which,  when  they  are  very  young,  structures  appear  that 
suggest  ancestors  that  lived  in  the  water.  However  that 
may  have  been,  it  is  very  certain  that  water  is  one  of  the 
largest  factors  in  life  to-day,  and  to  get  a  pure  water- 


RELATIONS  BETWEEN  WATER  AND  LIFE          9 

supply  is  one  of  the  first  problems  men  must  solve,  whether 
they  live  in  city  or  country. 

In  physiology  you  learned  about  digestion.  You  know 
that  digestion  is  the  changing  of  food  into  liquid  form. 
All  food  must  be  so  changed  before  our  bodies  can  use  it, 
md  the  basis  of  this  liquid  form  of  food  is  water.  Nearly 
all  of  the  weight  of  blood  is  water.  Plants,  in  proportion 
to  their  weight,  need  even  more  water  than  do  animals, 
especially  when  they  are  growing.  Their  roots  are  always 
taking  in  water  from  the  soil,  and  from  their  leaves  water 
is  always  going  off  into  the  air.  Water  evaporates  from 
leaves  just  as  it  evaporates  from  the  surface  of  lakes  and 
ponds. 

When  we  say  "water"  we  think  of  a  liquid.  But  we 
know  that  water  below  a  certain  temperature  changes  to  a 
solid  that  we  call  ice.  And  above  a  certain  temperature  it 
changes  to  a  gas  that  we  call  steam.  It  may  also  change 
into  an  invisible  gas  that  we  call  water  vapor.  This  change 
of  water  into  ice  when  it  is  cold,  and  into  steam  when  it  is 
hot,  suggests  that  heat  determines  the  form  in  which  sub- 
stances exist.  Thus  you  see  you  can  hardly  begin  to  study 
one  of  these  topics  before  you  begin  to  study  another.  We 
have  just  begun  to  study  water  and  already  we  need  to 
know  something  about  heat.  Just  what  is  heat?  That 
is  certainly  a  question  we  shall  have  to  answer  before  we 
•get  very  far  in  our  study  of  water. 

There  are  three  principal  forms  in  which  substances  exist: 
ihe  solid  form,  the  liquid  form,  and  the  gaseous  form.  Scien- 
tists refer  to  all  substances  taken  together  as  matter,  and 
they  call  solid,  liquid,  and  gas  three  states  of  matter. 
The  "solid  earth"  on  which  we  walk  is  the  most  familiar 
solid,  water  the  most  familiar  liquid,  and  air  the  most 


io  ELEMENTARY  SCIENCE 

familiar  gas.  Our  lives  are  so  very  closely  related  to  all 
three  of  these  that  it  is  impossible  to  say  which  one  of 
them  is  most  important  to  us. 

Now  let  us  think  of  the  different  ways  in  which  the 
changes  of  water  from  one  state  to  another  affect  our  lives. 
First,  the  change  from  liquid  to  gas.  You  know  that  boil- 
ing water  gives  off  steam,  and  you  know  that  even  cool 
water  gradually  evaporates,  which  means  that  it  changes 
into  water  vapor.  There  are  at  least  three  ways  in  which 
this  changing  of  water  from  liquid  to  gas  affects  our  lives. 
Two  very  important  things  in  our  lives  are  rain  and  steam- 
engines.  Both  of  these  depend  upon  the  changing  of  water 
from  a  liquid  into  a  gas.  Can  you  explain  how  ? 

Every  boy  or  girl  probably  knows  that  w hen  water  changes 
into  steam  it  expands,  and  expands  with  much  force.  Prob- 
ably you  have  heard  about  James  Watt,  the  Scotchman 
who  invented  the  steam-engine  about  one  hundred  and 
fifty  years  ago.  The  story  is  told  that  once  when  he  was 
a  boy  he  stuck  a  cork  in  the  spout  of  a  teakettle,  and  the 
steam  blew  it  out.  It  blew  it  out  so  hard  that  he  began 
to  think  about  the  force  behind  the  blow,  and  wondered  if 
that  force  could  be  used  in  any  useful  way.  That  was  the 
idea  that  led  finally  to  the  invention  of  the  steam-engine. 
In  all  steam-engines  the  fresh  steam  enters  a  strong  cylinder 
where  it  can  expand  only  by  pushing  a  piston,  just  as  the 
steam  in  the  spout  of  the  kettle  pushed  the  cork.  One  end 
of  a  steel  rod  is  attached  to  the  piston,  and  the  other  end  to 
a  wheel.  As  the  piston  is  pushed  to  and  fro  by  the  pres- 
sure of  the  steam,  the  rod  turns  a  wheel,  and  thus  the 
machinery  is  operated  (see  Fig.  i).  You  can  see  the 
cylinders  and  the  piston-rods  and  the  driving-wheels  on 
any  locomotive. 


RELATIONS  BETWEEN  WATER  AND  LIFE        n 

The  third  way  in  which  the  changing  of  water  from  liquid 
into  gas  affects  us  is  by  keeping  us  cool  in  hot  weather.  This 
is  not  quite  so  easy  to  understand,  and  yet  it  is  a  thing 
that  has  happened  to  you  hundreds  of  times.  Why  is  it 
that  it  cools  your  face  to  fan  it  ?  Chiefly  because  the  stir- 
ring of  the  air  makes  the  moisture  on  your  face  evaporate 
more  rapidly.  As  the  tiny  particles  pass  into  the  air, 


FIG.  i. — Principle  of  the  steam-engine:  fire  at  H  heats  water  in  boiler  A;  this 
produces  steam  which  travels,  as  indicated,  into  cylinder  A',  pushes  piston  P, 
and  operates  the  machinery  to  which  the  engine  is  attached. 

they  carry  heat  away  with  them.  Evaporation  is  a  cooling 
process.  This  is  a  matter  we  shall  have  to  discuss  more 
fully  a  little  later.  Here  again  we  find  that  we  cannot 
study  water  without  studying  heat. 

Now  about  the  changing  of  water  from  liquid  to  solid. 
That  is  what  gives  us  ice,  and  it  would  be  very  inconvenient 
for  us  not  to  have  ice.  Then  there  is  another  interesting 
thing  about  the  changing  of  water  from  liquid  to  solid, 
a  thing  that  affects  our  lives,  a  thing  that  seems  to  be  an 
exception  to  one  of  the  "rules"  of  nature.  That  rule  is 
that  heat  causes  expansion  and  cold  causes  contraction. 


12  ELEMENTARY  SCIENCE 

Nearly  all  substances  expand  when  they  are  heated,  and 
contract  when  they  are  cooled.  But  in  the  case  of  water 
this  applies  only  up  to  a  certain  point.  Water  contracts 
as  it  cools  nearly  up  to  the  point  when  ice  is  formed,  and 
then  it  expands.  If  it  did  not,  all  the  fish  in  our  streams 
and  lakes  would  be  killed.  Can  you  see  why  this  is  ?  Sup- 
pose ice  was  contracted  water  instead  of  expanded  water. 
Then  a  cubic  foot  of  ice  would  weigh  more  than  a  cubic 
foot  of  water,  and  all  the  ice  would  form  at  the  bottoms 
of  streams  and  lakes  instead  of  at  the  surface.  As  it  is,  the 
ice  on  top  really  protects  the  water  beneath  from  becoming 
cold  enough  to  freeze.  If  the  ice  began  to  form  at  the  bot- 
tom, our  lakes  and  streams  would  all  be  solid  ice  before  the 
winter  was  over.  The  only  inconvenient  thing  about  the 
expansion  of  water  when  it  turns  to  ice  is  that  sometimes 
this  causes  the  bursting  of  unprotected  water-pipes. 

These  things  that  you  have  been  reading  about  are  very 
easy  to  see,  but  they  are  not  so  easy  to  explain.  For  hun- 
dreds and  hundreds  of  years  men  observed  these  things 
and  were  unable  to  explain  them.  But  now,  in  a  very  short 
time,  you  can  learn  explanations  of  things  that  your  great- 
grandfathers could  not  explain  at  all.  Take  this  matter 
of  evaporation  that  is  so  important  to  our  lives.  You 
know  what  it  is,  but  how  are  you  going  to  explain  it  ?  Here 
is  a  dish  of  water.  You  know  that  after  a  while,  even 
when  nobody  touches  it,  the  water  will  be  gone.  It  will 
pass  into  the  air.  How  did  this  happen?  To  understand 
this  you  must  understand  that  water  and  all  other  sub- 
stances are  composed  of  very,  very  small  parts,  so  small 
that  they  are  invisible.  These  invisible  and  very  small 
parts  of  substances  are  called  molecules.  Though  they  are 


RELATIONS  BETWEEN  WATER  AND  LIFE        13 

far  too  small  to  be  seen,  scientists  have  been  able  to  mea- 
sure the  size  of  molecules.  The  water  molecule  is  so  small 
that  there  are  millions  of  them  in  a  single  drop.  Since 
they  are  so  wonderfully  small,  it  is  not  hard  to  understand 
that  they  may  fly  upward  from  the  surface  of  a  liquid. 
The  surface  of  a  liquid  may  seem  to  be  very  quiet,  yet 
something  is  going  on  there  all  the  time. 

Think  of  the  surface  of  a  pond  on  a  summer  day.  The 
warm  sun  shines  on  it.  The  air  above  it  is  dry.  A  breeze 
blows  over  it.  Under  these  conditions  evaporation  is 
rapid.  But  if  the  air  were  cold  and  moist  and  still,  evapora- 
tion would  be  very  slow.  You  see  the  air  can  hold  just 
so  much  water  and  no  more.  We  say  of  a  sponge  that  is 
full  of  water  that  it  is  saturated.  Air  that  has  taken  up 
all  the  water  it  can  is  also  said  to  be  saturated. 

But  to  go  back  to  our  pond.  The  heat  and  the  wind 
and  the  dryness  of  the  air  make  millions  upon  millions  of 
invisible  molecules  keep  rising  from  the  pond.  Up  and 
up  they  go,  until  at  last  they  come  where  the  air  is  cooler. 
Then  what  happens?  Then  these  tiny  molecules  of  water 
begin  to  condense.  It  is  as  though  they  huddled  together 
in  crowds.  These  crowds  of  molecules  are  tiny  droplets, 
so  small  that  you  could  not  see  one  of  them,  and  yet  when 
billions  and  billions  of  them  are  near  each  other  they  form 
what  we  call  clouds. 

Here  for  the  third  time  in  studying  water  we  have  come 
to  heat.  Cold  means  simply  less  heat,  and  you  have  just 
noted  that  heat  causes  water  molecules  to  evaporate,  and 
then  less  heat  causes  them  to  condense  again.  If  it  were 
not  for  heat,  all  the  water  in  the  world  would  change  to 
ice,  and  would  remain  in  that  form.  Water  seems  to  be 
quite  an  active  substance,  changing  constantly  as  it  does 


I4  ELEMENTARY  SCIENCE 

from  one  form  to  another.  But,  really,  water  is  not  active. 
It  is  heat  that  is  active.  Water  is  passive,  and  it  is  changes 
in  the  amount  of  heat  that  are  responsible  for  the  changes 
of  water.  Water  is  a  sort  of  tool,  and  heat  is  the  great 
force  that  uses  that  tool.  We  say  that  the  heat  of  the 
sun  draws  water  up.  It  changes  from  liquid  to  gas.  Then 
it  changes  from  gas  back  to  liquid  again,  and  falls  as  rain. 
It  keeps  moving  and  changing  all  the  time.  It  goes  down 
deep  into  the  soil  and  into  crevices  in  the  rock,  and  then 
again  it  rises  in  the  air  far  above  the  earth.  Water  is  a 
wonderful  traveller.  It  is  in  the  bodies  of  plants  and 
animals  as  well  as  in  the  seas  and  lakes,  and  it  travels 
from  the  fields  of  ice  and  snow  in  the  far  north  and  south 
down  to  the  tropics  again.  Yet  with  all  this  movement 
and  changing  of  form  there  is  always  practically  the  same 
amount  of  water  in  the  world.  It  may  change  its  form  and 
its  place,  but  it  is  not  destroyed. 

So,  even  by  studying  water  alone  we  learn  that  we  live 
in  a  very  changing  world.  Our  surroundings  are  con- 
stantly changing,  just  as  we  ourselves  are  constantly 
changing. 

We  do  not  need  to  travel  to  see  these  changes.  We  can 
stay  where  we  are  and  watch  day  change  into  night.  We 
can  watch  the  clouds  marching  across  the  sky,  and  the 
storms  as  they  gather  and  break.  We  can  feel  the  wind 
and  the  rain  in  our  faces,  and  can  see  the  rivers  running  to 
the  sea.  We  watch  the  procession  of  the  seasons,  winter 
changing  to  spring,  and  autumn  to  winter  again.  Each 
day  the  country  is  a  little  different  from  what  it  was  the 
day  before.  The  plants  and  the  animals  change  as  well  as 
their  surroundings.  Each  week  of  the  spring  and  summer 
months  you  will  find  different  birds  and  insects  in  the 


RELATIONS  BETWEEN  WATER  AND  LIFE        15 

fields  and  woods.  Yesterday,  when  I  looked  from  my  win- 
dow, the  whole  surface  of  the  earth  was  green  as  far  as  I 
could  see.  But  to-day  it  is  beginning  to  turn  brown,  be- 
cause the  blue  grass  is  ripening. 

You  saw  that  it  is  heat  that  causes  the  changes  and 
movements  of  water.  And  it  is  changes  in  the  amount  of 
heat  that  cause  the  seasons  of  the  year.  You  can  see  that 
this  would  be  a  very  still  and  dead  world  indeed  if  it  were 
not  for  heat. 

QUESTIONS 

1.  How  much  of  the  earth  is  covered  by  water? 

2.  What  is  the  purpose  of  digestion  ? 

3.  What  determines  the  three  states  of  water? 

4.  What  is  the  force  that  drives  the  steam-engine? 

5.  Why  does  it  cool  your  face  to  fan  it? 

6.  How  does  the  size  of  a  substance  change  when  it  is  heated? 

7.  Why  do  water-pipes  burst  in  winter? 

8.  What  are  clouds? 


CHAPTER  III 
THE  DISTRIBUTION  OF   WATER 

The  distribution  of  water  is  evidently  very  important. 
We  must  find  out  about  it.  What  is  it  that  keeps  it  mov- 
ing all  over  the  surface  of  the  earth?  Does  it  move  of 
itself,  or  does  something  else  move  it? 

There  are  four  things  that  explain  the  distribution  of 
water.  These  four  things  are  evaporation,  condensation,  air 
movements,  and  gravity.  Evaporation  explains  how  water 
gets  up  into  the  air.  Condensation  explains  how  clouds 
are  formed.  Air  movements  explain  why  the  clouds  move 
about  in  the  sky.  Gravity  explains  why  the  rain  falls  to 
the  earth,  and  why  the  water  goes  down  to  the  roots  of 
plants,  and  why  the  streams  flow  to  the  seas.  Here  are 
four  things  that  are  at  work  all  the  time.  Night  and  day 
they  keep  the  water  moving.  Up  it  goes  in  invisible  mist 
that  changes  to  visible  clouds.  Over  sea  and  land  the 
clouds  are  always  moving,  and  somewhere  the  water  is  al- 
ways falling  back  to  earth  again  as  rain.  Always  the  water 
that  falls  as  rain  is  going  into  the  soil  and  into  the  streams. 
In  the  soil  some  of  it  enters  the  roots  of  plants,  and  then 
it  passes  out  from  their  leaves  into  the  air  again.  Without 
this  movement  of  water  through  their  bodies,  the  plants 
would  soon  wither  and  die.  Water  lets  them  live  and 
grow. 

Evaporation.  —  If  you  have  a  bird-bath  in  your  yard, 
you  know  that  it  is  hard  to  keep  it  filled.  The  water  evap- 

16 


THE  DISTRIBUTION  OF  WATER  17 

orates.  On  wash-day  wet  clothes  are  put  out  on  the  line 
and  presently  they  are  dry.  You  can't  see  the  water  go, 
but  you  know  that  it  has  gone.  How  are  we  going  to  ex- 
plain this? 

A  drop  of  water  is  composed  of  millions  of  particles.  It 
is  impossible  to  see  these  particles,  but  we  know  that  they 
are  there  by  the  way  they  behave.  These  particles  are 
always  in  motion.  They  move  in  straight  lines.  They 
strike  each  other,  and  rebound.  At  the  surface  they  are 
always  flying  off  into  the  air.  It  is  this  flying  off  of  the 
particles  of  water  that  we  call  evaporation.  There  is  al- 
ways some  water  in  the  air  in  the  form  of  these  invisible 
particles.  When  there  is  a  good  deal  of  water  in  the  air 
we  say  that  it  is  moist  or  humid. 

Evaporation  does  not  always  go  on  at  the  same  rate. 
Sometimes  it  is  rapid,  sometimes  slow.  The  more  moisture 
there  is  in  the  air  the  slower  the  rate  of  evaporation.  Heat 
and  wind  affect  it.  It  is  most  rapid  when  the  air  is  hot  and 
dry  and  the  wind  is  blowing.  It  is  slowest  when  the  air 
is  moist  and  cold  and  there  is  no  wind. 

Evaporation  is  a  cooling  process.  As  the  water  passes 
away,  heat  passes  away  with  it.  On  hot  days  if  the  air  is 
dry,  we  are  much  more  comfortable  than  if  it  is  moist. 
This  is  because  rapid  evaporation  of  moisture  from  our 
bodies  keeps  us  cool.  But  if  the  air  is  moist  as  well  as  hot, 
we  are  very  uncomfortable.  Moisture  stays  on  our  skin, 
and  heat  stays  with  it. 

Evaporation  goes  on  all  over  the  world  all  the  time. 
All  the  time  water  is  passing  into  the  air  from  all  the  seas 
and  lakes  and  streams,  and  from  all  the  leaves  of  plants. 
Each  year  enough  rain  falls  to  make  a  sheet  of  water  be- 
tween thirty  and  forty  inches  deep  all  around  the  world, 


zg  ELEMENTARY  SCIENCE 

and  this  shows  how  much  water  evaporates.  For  all  the 
water  that  falls  as  rain  must  have  gone  up  by  evaporation. 
There  is  no  other  way. 

Condensation.  —  After  the  water  particles  get  up  in  the 
air,  a  number  of  things  may  happen  to  them.  At  first 
they  are  lighter  than  the  air  and  keep  moving  up,  some- 
what as  a  balloon  moves  up.  But  presently  they  come  to 
where  the  air  is  cooler.  The  cool  air  affects  them.  They 
begin  to  crowd  together.  This  is  what  is  called  condensa- 
tion. The  particles  condense  into  small  drops.  You  can 
see  how  this  happens  by  holding  a  cool  piece  of  glass  in 
the  steam  that  comes  from  the  spout  of  a  kettle. 

We  cannot  see  the  particles  of  water  when  they  evapo- 
rate. They  are  much  too  small.  But  after  they  have  con- 
densed into  little  drops  they  form  clouds.  When  the  sun 
is  shining  on  them  the  clouds  are  white  and  beautiful. 
They  drift  with  the  wind.  When  a  cloud  forms  close  to 
the  surface  of  the  earth  we  call  it  fog. 

Sometimes  the  air  just  over  the  earth  is  moist  when 
evening  comes.  Then,  if  the  night  is  cool,  this  moisture 
condenses  into  little  drops  that  we  see  shining  on  the  grass 
when  the  sun  comes  up.  We  call  it  dew.  If  the  night  is 
so  cold  that  this  moisture  condenses  in  the  form  of  tiny 
crystals  of  ice,  we  call  it  frost. 

The  tiny  drops  of  water  that  form  the  clouds  are  no 
heavier  than  the  air,  so  they  stay  up  in  the  air  and  float 
along  like  tiny  balloons.  But  sometimes  they  come  to 
where  it  is  cooler.  Then  they  condense  more  and  more, 
until  they  form  big  drops  that  are  heavier  than  air.  The 
drops  begin  to  fall.  The  clouds  are  breaking  up  in  rain. 

The  warmer  the  air  the  higher  the  light  clouds  go  up  in 


THE  DISTRIBUTION  OF  WATER  19 

the  sky.  The  heat  keeps  the  tiny  drops  from  condensing. 
Then,  suddenly,  these  high  clouds  may  come  into  air  that  is 
very  cold,  so  cold  that  it  makes  their  drops  condense  quickly 
in  the  form  of  ice.  The  drops  of  ice  plunge  down  to  earth, 
getting  bigger  as  they  fall.  These  drops  of  ice  we  call  hail. 

Air  Movements.  —  Now  you  understand  how  the  water 
gets  up  into  the  air.  The  next  thing  is  to  understand  how 
it  gets  carried  over  the  land.  The  winds  blow  the  clouds, 
you  say.  Yes,  but  what  is  it  that  makes  the  winds  blow  ? 
Air  does  not  move  unless  there  is  something  that  makes  it 
move.  What  we  want  to  find  out  is  the  cause  of  air  move- 
ments. 

Air  is  always  moving.  It  is  the  most  actively  moving 
substance  in  the  world.  It  may  move  in  any  direction. 
Its  horizontal  movements  we  call  wind.  But  there  are  also 
up  and  down  movements  that  we  do  not  feel.  As  it  moves, 
the  air  carries  with  it  both  heat  and  moisture.  You  know 
how  important  the  circulation  of  the  blood  is  to  your  body. 
The  circulation  of  the  air  is  just  about  as  important  for 
the  earth  as  the  circulation  of  the  blood  is  for  your  body. 
It  is  the  beating  of  your  heart  that  keeps  your  blood  mov- 
ing. What  is  it  that  keeps  air  moving? 

Hot  air  is  lighter  than  cold  air.  You  have  learned  about 
this  in  geography.  You  know  that  the  heat  of  the  sun  is 
greater  near  the  equator  than  it  is  at  the  poles.  So  the 
warm  air  near  the  equator  rises  and  flows  away  toward  the 
poles,  while  the  cool  air  from  the  north  and  south  keeps 
flowing  in  toward  the  equator,  under  the  warmer  air. 
Thus  the  heat  of  the  sun  starts  the  circulation  of  earth's 
atmosphere,  just  as  the  heat  of  a  stove  starts  a  circulation 
of  air  in  a  room. 


2O 


ELEMENTARY  SCIENCE 


So  the  sun  explains  how  the  air  starts  to  circulate.  But 
that  is  not  enough.  We  want  to  know  about  the  winds 
that  blow  from  every  direction.  Sometimes  they  are  very 
gentle  and  pleasant,  sometimes  they  are  very  strong  and 
destructive.  What  makes  the  air  behave  this  way? 

The  air  behaves  in  this  way  because  it  is  always  getting 
stirred  up,  and  then  tries  to  smooth  itself  out  again.  A 
high  wind  is  just  the  rushing  in  of  a  lot  of  air  to  take  the 
place  of  other  air  that  has  moved  away.  The  thing  that 

keeps  the  atmosphere 
stirred  up  is  the  heat 
of  the  sun.  You  know 
how  heat  keeps  the 
water  in  a  kettle 
stirred  up.  Somewhat 
in  the  same  way  the 
heat  of  the  sun  keeps 
the  air  stirred  up. 

The  heat  of  the  sun 
comes  down  on  the 
air,  and  the  air  catches 
some  of  this  heat  and 

holds  it.  The  air  is  a  sort  of  trap  for  heat.  But  some 
parts  of  the  world  get  more  heat  than  other  parts.  Also 
the  amount  of  heat  received  from  the  sun  varies  at  dif- 
ferent times  of  day.  So,  always,  some  parts  of  the  air  get 
hotter  than  other  parts,  and  that  is  what  keeps  the  air 
moving.  For,  just  as  soon  as  any  air  gets  warmer  than 
the  air  around  it,  it  begins  to  move.  It  begins  to  move 
up,  because  it  is  lighter  than  the  rest  of  the  air  (see  Fig.  2). 
Now,  because  the  air  is  always  stirred  up  in  this  way, 
the  amount  of  air  over  your  head  keeps  changing.  If  a 


FIG.  2.— Diagram  to  illustrate  how  the  air  be- 
comes heated  at  the  equator  and  moves  up- 
ward.— After  SALISBURY. 


THE  DISTRIBUTION  OF  WATER  21 

good  deal  of  the  air  above  us  has  moved  away,  we  have 
what  is  called  low  pressure,  that  is,  a  low  pressure  of  air 
on  the  surface  of  the  earth.  But  if  the  air  is  piled  up 
higher  than  usual  in  some  places,  those  places  have  high 
pressure.  Then  what  happens?  It  is  then  that  the  winds 
begin  to  blow.  The  air  seems  to  try  to  get  evenly  distrib- 
uted again.  It  rushes  from  the  high-pressure  places  to 
the  low-pressure  places.  The  greater  the  difference  in 
pressure,  the  faster  the  air  will  move.  It  is  like  water 
rushing  down  a  steep  slope.  It  may  rush  so  fast  that  there 
is  a  great  storm.  So  the  air  is  always  trying  to  smooth  it- 
self out  (trying  to  equalize  its  pressures),  and  the  sun,  by 
heating  it  unevenly,  is  always  getting  it  stirred  up  again. 
As  a  result  of  these  two  things,  the  air  is  always  moving 
and  always  distributing  heat  and  moisture  over  the  sur- 
face of  the  earth. 

You  will  learn  a  good  deal  more  about  this  when  we 
study  the  weather.  But  now  you  have  learned  enough  to 
understand  how  it  is  that  water  gets  distributed.  It  is  the 
heat  of  the  sun  that  does  it.  It  is  the  heat  of  the  sun  that 
starts  evaporation,  and  it  is  the  heat  of  the  sun  that  keeps 
the  air  moving.  So  it  is  the  sun  that  keeps  both  air  and 
water  moving.  It  is  the  sun,  if  anything,  that  is  the  heart 
of  the  world,  and  the  air  and  water  that  the  sun  moves 
sustain  life  just  as  truly  as  the  blood  that  your  heart  moves 
sustains  life  in  your  body. 

Gravity.  —  The  fourth  thing  that  explains  the  movements 
of  both  air  and  water  is  gravity.  It  is  heat  that  explains 
why  air  and  water  go  up.  It  is  gravity  that  explains  why 
they  come  down  again.  We  must  understand  both  of 
these  things  and  see  how  they  work  together. 


22  ELEMENTARY  SCIENCE 

Gravity  is  just  as  important  as  any  of  the  other  great 
things  that  we  have  started  out  to  study.  It  has  done  just 
as  much  as  any  of  them  in  making  the  earth  so  that  we 
can  live  on  it. 

Gravity  is  a  property  or  quality  of  the  earth.  It  is  not 
a  thing  that  we  can  see  or  feel;  it  is  a  "law  of  nature." 
It  is  the  "pull"  of  the  earth  upon  all  things  on  its  surface 
or  near  it.  It  is  the  power  that  makes  an  apple  fall  to  the 
ground,  and  makes  water  run  down-hill.  It  is  the  power 
that  makes  it  easier  for  us  to  go  down  than  to  go  up.  It 
is  the  power  that  keeps  water  and  air  from  flying  off  into 
space.  It  is  the  power  of  Mother  Earth  to  hold  things  to 
her  surface. 

There  is  a  law  of  nature  that  all  solid  bodies  have  power 
to  attract  other  bodies  toward  them.  Thus  it  is  the  at- 
traction of  the  moon  that  causes  tides  in  the  sea.  This 
law  is  called  the  law  of  gravitation.  Gravity  is  simply  the 
law  of  gravitation  as  applied  to  the  earth.  No  one  has 
explained  gravitation.  Gravitation  explains  other  things, 
but  it  does  not  explain  itself.  It  is  one  of  those  things  that 
you  will  have  to  accept  without  any  explanation. 

This  is  just  a  beginning  of  the  study  of  water.  You  have 
learned  an  explanation  of  its  movement.  You  have  found 
that  heat  and  gravity  are  the  chief  causes  of  the  movements 
of  both  air  and  water.  Now  we  must  see  what  the  water 
does  as  it  moves  about  on  the  surface  of  the  earth. 


QUESTIONS 

1.  How  is  water  distributed? 

2.  What  is  "humid"  air? 

3-  When  is  evaporation  most  rapid? 


THE  DISTRIBUTION  OF  WATER  23 

4.  Why  are  we  more  uncomfortable  on  moist  hot  days  than  on 

dry  hot  days? 

5.  What   happens  if  you  hold  a  cold  piece  of  glass  in  steam? 

6.  What  is  fog? 

7.  What  are  dew  and  frost  and  hail? 

8.  How  is  hail  formed? 

9.  Why  does  the  wind  blow? 
10.  Define  gravity. 


CHAPTER  IV 
EFFECTS  OF  THE  MOVEMENTS  OF    WATER 

You  have  seen  that  water  is  a  great  tool  of  nature,  and 
that  heat  and  the  force  of  gravity  are  the  forces  that  work 
with  this  tool.  Air  is  another  great  tool  of  nature,  and  it, 
too,  is  moved  by  heat  and  by  the  force  of  gravity.  Up  and 
down,  across  the  surface  of  the  land  and  under  it,  these 
tools  keep  moving  and  working.  The  winds  keep  blowing, 
rains  keep  falling,  and  water  keeps  running  back  to  the 
sea.  What  effects  do  these  things  have  upon  us? 

As  air  and  water  move  they  move  other  things  with 
them.  Things  that  are  loose  are  shifted  about.  High 
winds  blow  dust  and  sand  from  place  to  place,  and  high 
water  sweeps  along  everything  in  its  path  that  is  not  firmly 
anchored.  Thus  some  shifting  of  the  materials  of  the 
earth's  surface  goes  on  all  the  time,  and  running  water  is 
by  far  the  most  important  of  the  shifters  of  materials. 
Water  is  eighty  times  as  heavy  as  air;  so  it  can  carry  eighty 
times  as  much.  Every  muddy  stream  is  carrying  a  load 
that  will  be  laid  down  again  somewhere.  This  is  called 
the  wash  or  the  waste  of  the  land.  The  Mississippi  River 
carries  out  to  sea  a  million  tons  of  mud  and  sand  each  day. 
(How  many  freight-trains  of  fifty  cars  each  would  be  re- 
quired to  carry  this  load,  allowing  five  tons  to  the  car?) 

Running  water  plays  an  important  part  in  the  lives  of 
men.  It  does  both  good  and  harm.  The  rivers  carry 
away  to  the  sea  much  good  fertile  soil,  but  they  also  bring 
good  soil  down  from  the  hills  and  spread  it  along  their 


EFFECTS  OF  THE  MOVEMENTS  OF  WATER      25 

courses.  In  geography  you  learned  of  the  rich  soil  of  the 
river-bottoms.  This  soil  is  composed  of  materials  carried 
and  distributed  by  flooded  rivers.  Rivers  in  flood  some- 
times destroy  the  work,  and  even  the  lives  of  men,  but  all 
the  time  they  are  turning  thousands  of  water-wheels  that 
do  work  for  him,  and  they  make  a  highway  for  his  boats. 
Rivers  have  been  of  great  importance  in  the  history  of 
man.  There  was  a  time  when  nine-tenths  of  all  the  people 
in  the  world  lived  on  rivers.  This  was  because  of  the  fer- 
tility of  the  soil  there,  and  because  of  the  ease  of  travel 
by  boats. 

Erosion  and  deposition  are  two  scientific  terms  that  de- 
scribe the  process  of  shifting  materials  on  the  earth's  sur- 
face. Erosion  means  the  "gnawing  away"  of  material. 
We  say  that  acids  erode  metals;  they  seem  to  gnaw  them 
away.  So,  also,  we  say  that  wind  and  water  erode  the 
land,  especially  the  bare,  uncovered  parts  of  the  land. 
The  waves  gnaw  away  at  the  shores,  and  the  streams  gnaw 
away  at  the  hills.  And  all  the  material  that  is  eroded  is 
deposited  somewhere  again. 

Protection  by  Plants.  —  All  this  sounds  as  though  the 
world  was  a  very  uncertain  place  in  which  to  live.  It 
sounds  as  though  the  land  on  which  we  live  might  be 
washed  away  from  beneath  our  feet.  Of  course  this  is  not 
true,  but  it  would  be  true  if  it  were  not  for  the  natural 
covering  and  protection  of  the  land  that  is  formed  by  the 
plants,  especially  the  grass  plants.  In  places  where  plants 
cannot  grow  erosion  may  be  very  rapid.  Think  of  the 
"shifting  sands"  of  the  Sahara  Desert,  or  of  the  "bad 
lands"  in  our  own  country.  In  these  bad  lands,  where 
plants  do  not  protect  the  surfaces,  it  is  plain  to  see  that 


26  ELEMENTARY  SCIENCE 

erosion  is  a  sort  of  gnawing  process;  the  sharp  hills  look 
as  though  their  sides  had  been  gnawed.  On  railroad  em- 
bankments it  is  very  important  to  start  the  growth  of 
tough  grass,  whose  roots  hold  the  soil  together,  and  keep 
it  from  washing  away.  If  the  plants  were  gone  it  would 
not  be  long  before  the  whole  surface  of  the  land  would 


FIG.  3.— Gully  cut  t 


begin  to  look  something  like  the  bad  lands  of  South  Da- 
kota. As  it  is,  our  protected  soil  is  not  washed  away 
much  except  where  streams  gnaw  away  at  their  banks, 
working  under  the  soil  and  trees,  or  where  gullies  are 
formed,  or  where  there  are  steep  slopes. 

A  Picture  of  the  Wearing  Down  of  the  Land.  —  Now,  in 
mind's  eye,  you  can  begin  to  see  the  way  in  which  water 
wears  down  the  land.  You  can  see  the  streams,  swollen 


EFFECTS  OF  THE  MOVEMENTS  OF  WATER      27 

by  heavy  rains,  rushing  down  the  sides  of  hills  and  moun- 
tains, tumbling  the  boulders  along  as  they  go.  You  can 
see  the  waves  of  lakes  and  oceans,  beating  against  their 
shores,  and  grinding  rock  into  finer  and  finer  material. 
After  a  storm,  you  have  seen  the  washouts  along  the 
roadsides,  and  the  gullies  that  are  cut  back  into  the  fields 
and  hillsides  (see  Fig.  3).  You  can  see,  too,  that  men  have 


FIG.  4.— View  showing  how  the  roots  of  plants  hold  the  soil  and  prevent 
it  from  being  washed  away  by  the  rainfall. 

done  much  work  to  keep  the  streams  in  their  courses,  and 
to  prevent  washouts. 

Here  are  two  very  simple  facts.  Water  runs  down-hill, 
and  as  it  runs,  it  carries  things  with  it.  It  is  just  these  sun- 
pie  facts  that  have  determined  the  forms  of  the  land. 
Water  running  down-hill  is  the  great  sculptor  of  nature. 
Slowly  but  surely  the  whole  surface  of  the  land  is  being 
brought  down  to  sea-level.  Of  course  it  is  a  very  uneven 
process;  erosion  is  much  more  rapid  at  some  places  than  at 
others,  but  it  goes  on  all  the  time.  Where  slopes  are  steep, 


28 


ELEMENTARY  SCIENCE 


and  heavy  rains  are  frequent,  and  rock  and  soil  are  loose, 
erosion  is  rapid.  Where  the  land  is  level  and  well  covered 
with  plants,  erosion  is  very  slow  (see  Fig.  4).  But  slow 
or  fast,  the  result  is  the  same.  The  land  is  being  worn 
down  to  the  sea.  As  long  as  there  is  a  bit  of  soil  or  rock 
that  stands  even  one  foot  above  sea-level,  wind  and  water 

will  attack  it.  They 
will  work  at  it  till  they 
wear  it  down  (see  Figs. 
5  and  6). 

The  Rise  of  the  Land. 
-"But,"  you  ask,  "if 
all  this  is  true,  why 
have  we  any  hills  and 
mountains  left?  Why 
is  not  the  whole  surface 
of  the  land  worn  down 
nearly  to  sea-level?" 

If  you  have  ever  felt 
an  earthquake  or  seen  a  volcano  you  can  probably  guess 
the  answer.  Earthquakes  and  volcanoes  show  us  that  in- 
side the  earth  great  forces  exist.  These  forces  are  strong 
enough  to  lift  islands  out  of  the  sea,  or  gradually  to  raise 
up  whole  ranges  of  mountains.  They  have  done  this  in 
the  past,  and  in  some  places  they  are  still  doing  it  to-day. 
Great  though  gentle  movements  of  the  land  seem  to  be 
going  on  all  the  time. 

Some  hundreds  of  years  ago  an  invading  army  landed  on 
the  east  coast  of  England.  The  town  at  which  it  landed 
is  now  miles  inland,  so  much  has  the  land  risen  since  that 
time.  There  are  many  such  places  in  the  world.  Also 


FIG.  5. — Diagram  showing  how  running  water 
cuts  into  the  land  and  washes  it  away,  until 
it  is  finally  lowered  almost  to  sea-level. 


EFFECTS  OF  THE  MOVEMENTS  OF  WATER       29 

there  are  many  places  where  the  edges  of  continents  have 
sunk  below  the  sea.  This  is  true  near  New  York  City. 
The  valley  of  the  Hudson  River  is  what  is  called  a  "  drowned 
valley";  a  long  time  ago  it  sank  below  the  sea-level,  so 
that  now  sea-water  mingles  with  the  river-water  for  many 
miles  above  the  mouth.  Careful  measurements  have 


FlG.  6.— Photograph  showing  how  running  water  cuts  away  the  softer  parts  of 
the  land  and  leaves  the  harder  rocks  sticking  up. 

proved  that  the  coast  of  Massachusetts  is  sinking  very 
slowly.  You  have  seen  how  air  and  water  keep  moving, 
and  now  you  learn  that  the  solid  part  of  the  earth  also 
keeps  moving.  Change  follows  change.  All  nature  is  just 
an  endless  procession  of  changes.  Not  even  the  hardest 
rock  is  quite  the  same  this  year  that  it  was  last  year. 

So  we  see  that,  while  running  water  keeps  wearing  the  land 
down,  hidden  forces  inside  the  earth  keep  lifting  it.  The  re- 
sult is  that  there  is  just  about  the  same  amount  of  dry 


3o  ELEMENTARY  SCIENCE 

land  to-day  that  there  was  when  man  first  began  to  live 
on  earth.  There  seems  to  be  no  danger  of  being  swallowed 
up  by  the  sea. 

Mountains  Worn  Down  and  Seas  Filled  Up.  —  Though 
the  amount  of  dry  land  may  have  changed  very  little  since 
human  history  began,  we  must  remember  that  human  his- 
tory is  a  very  small  thing  as  compared  with  the  whole  his- 
tory of  the  earth.  What  we  see  happening  to-day  to  the 
surface  of  the  earth  is  but  a  trifle  compared  with  what  has 


FIG.  7. — Diagram  of  layers  of  rock  that  were  laid  down  in  the  sea  and  are  now  found 
miles  away  from  the  sea. — After  SNYDER. 

happened  in  the  past.  Whole  ranges  of  mountains  have 
been  worn  down  by  the  ceaseless  work  of  water,  and  have 
been  carried  off,  grain  by  grain,  into  the  sea.  Seas  and 
lakes  have  been  filled  with  this  waste  of  the  land,  and  geol- 
ogists, who  study  the  rocks  and  find  in  them  a  history 
of  the  earth,  make  maps  which  show  that  the  continent 
of  North  America  was  once  just  a  series  of  long  and  narrow 
islands.  The  waste  of  these  islands  was  carried  down  into 
the  shallow  seas  that  surrounded  them.  The  mud  was 
pressed  down  by  the  weight  of  more  mud.  The  dead  bodies 
of  small  animals  that  lived  in  that  ancient  sea  were  buried 
in  this  mud.  It  hardened  into  rock.  After  a  great  while 
it  was  lifted  above  the  surface,  so  that  to-day,  hundreds  of 
miles  from  the  sea,  we  find  layers  of  rocks  that  were  formed 


EFFECTS  OF  THE  MOVEMENTS  OF  WATER      31 


in  the  sea  (see  Fig.  7),  and  have  in  them  imprints  of  the 
bodies  of  animals  that  lived  in  those  days.  These  imprints 
of  ancient  animals 
are  called  fossils  (see 
Figs.  8  and  9). 

The  average  rain- 
fall of  the  United 
States  each  year  is 
about  thirty  inches. 
It  is  estimated  that 
about  one-half  of 
this  water  evapo- 
rates, about  one- 
third  of  it  runs  off 
over  the  surface,  and  one-sixth  sinks  into  the  ground. 
Let  us  think  for  a  while  about  that  one-sixth  that  sinks 
into  the  ground. 

Ground- Water.  —  For    at   least   two   reasons,   ground- 
water  is  very  important  to  man:  (i)  it  nourishes  plants; 

(2)  it  supplies  wells 
and  springs.  We  all 
get  our  food  from 
plants,  and  over  hah" 
the  people  in  the 
world  get  their  drink- 
ing water  from  wells 
and  springs. 

Rain-water  finds 
its  way  into  all  the 
cracks  and  pores  in 
Flo.  9.— Fossil*.  the    soil,   but    the 


3  2 


ELEMENTARY  SCIENCE 


deeper  it  goes  the  smaller  are  the  cracks  and  pores.  At 
last  the  pressure  is  so  great  that  all  cracks  and  pores  are 
closed  up.  Probably  there  is  no  water  deeper  than  a 
few  miles  in  the  earth. 

When  wells  are  dug  water  is  sometimes  found  quite 
near  the  surface,  sometimes  far  below  it.  The  level  at 
which  water  begins  to  collect  in  the  well  is  called  the  ground- 


FIG.  10. — Diagram  showing  how  the  ground-water  level  (indicated  by  the  dotted 
line)  follows  the  curves  of  the  land. 

water  level,  or  the  water-table.  The  water-table  is  the  top 
of  the  water  that  is  under  the  ground.  The  top  of  the 
water  that  is  in  lakes  or  streams  is  level,  but  the  top  of 
the  water  that  is  in  the  ground  is  not  level.  It  follows  the 
curves  of  the  land,  and  rises  higher  than  the  tops  of  streams 
and  lakes.  This  you  can  understand  better  by  studying 
the  picture  (see  Fig.  10).  Between  rains,  the  water-table 
sinks,  for  the  ground-water  keeps  seeping  out  into  wells 
and  springs,  into  lakes  and  rivers. 

Under  the  ground  the  water  does  not  flow  evenly  in 
all  directions.  This  is  because  there  are  so  many  differ- 
ent kinds  of  rock  and  soil.  The  different  kinds  of  rock 
and  soil  are  found  usually  in  layers.  These  layers  are 


EFFECTS  OF  THE  MOVEMENTS  OF  WATER      33 

called  strata.  In  a  layer  of  sand  or  gravel  the  water 
moves  easily,  because  sand  and  gravel  are  full  of  little 
openings.  They  are  very  porous.  But  in  clay  water  moves 


FIG.  ii. — Diagram  illustrating  two  types  of  springs; 
try  to  explain  them;  the  springs  occur  at  S;  P  means 
porous  layer;  C  is  a  perpendicular  crack  in  the  layers 
of  rock. 

very  slowly.  Clay  packs  very  closely;  it  is  much  less 
porous  than  sand  and  gravel.  Then  there  are  layers  of 
solid  rock  that  also  obstruct  the  movement  of  water  in 
the  ground.  So  ground-water  moves  laterally,  more  than 
it  moves  vertically.  It  moves  along  the  more  porous  layers. 


FIG.  12. — Diagram  illustrating  the  artesian  well,  as  ex- 
plained in  the  text;  P  is  a  porous  layer  of  rock;  the 
well  is  at  W. 

This  explains  why  there  are  springs.  Where  porous 
layers  that  have  water  in  them  come  to  the  surface,  the 
water  runs  out  and  forms  springs  (see  Fig.  n).  This  also 
explains  artesian  wells.  Artesian  wells  are  wells  from  which 


34 


ELEMENTARY  SCIENCE 


the  water  flows  naturally,  without  pumping.  This  is  be- 
cause the  bottom  of  the  well  is  in  a  porous  layer  that  receives 
water  at  some  place  that  is  higher  than  the  top  of  the  well. 
The  receiving  place  may  be  many  miles  away  from  the 
well.  Suppose  the  top  of  the  well  is  five  hundred  feet 


FIG.  13. — Photograph  of  an  artesian  well. 

above  sea-level,  and  the  well  is  two  hundred  feet  deep.  It 
goes  down  through  layers  of  clay  and  rock  through  which 
the  water  cannot  escape  upward.  At  last  it  enters  a 
porous  layer  that  is  full  of  water.  Now  this  porous  layer 
extends  for  many  miles  in  every  direction,  but  everywhere 
it  is  covered  by  rock  and  clay  except  in  some  hills  that  are 
fifty  miles  away.  These  hills  are  formed  by  the  turning 
up  of  the  underground  layers  of  rock  and  clay.  Here  the 
porous  layer  comes  to  the  surface,  and  here  it  receives 


EFFECTS   OF  THE  MOVEMENTS  OF  WATER      35 

water  from  the  rain  and  from  the  little  streams  that  flow 
in  the  hills.  This  place  where  the  porous  layer  comes  to 
the  surface  is  one  thousand  feet  above  sea-level.  Now  you 
can  see  why  water  flows  out  of  the  top  of  such  a  well  (see 
Figs.  12  and  13). 

Much  of  the  water  that  sinks  into  the  ground  is  taken 
up  by  the  roots  of  plants,  carried  up  into  'their  leaves,  and 
there  it  evaporates.  Evaporation  from  leaves  is  called 
transpiration.  Growing  plants  need  to  have  water  passing 
constantly  through  them.  Their  bodies  are  chiefly  com- 
posed of  water,  and  the  water  that  moves  through  them 
brings  up  from  the  soil  substances  that  they  need  in  their 
growth.  In  the  growing  season  thousands  of  pounds  of 
water  transpire  every  day  from  an  acre  of  trees.  It  has 
been  estimated  that  corn  plants  transpire  about  five  thou- 
sand pounds  of  water  for  every  bushel  of  corn  that  they 
produce. 

Topography  means  the  form  of  the  land.  You  have 
learned  that  the  work  of  water  and  the  movements  of  the 
earth's  crust  are  the  principal  agents  in  determining  the 
nature  of  the  topography.  Upfoldings  of  the  earth's  crust 
have  formed  the  ranges  of  upland  and  mountains,  and  the 
work  of  water  has  deepened  the  valleys  and  smoothed  the 
contours  of  the  hills.  Together,  these  great  forces  of  nature 
have  formed  the  landscapes  that  to  us  are  so  beautiful. 
We  shall  find  them  all  the  more  beautiful  and  interesting 
if  we  know  something  of  the  forces  which  have  made  them, 
and  something  of  the  long  history  which  lies  behind  them. 

Glaciers.  —  You  have  noted  the  effects  of  running  water. 
But  water  in  the  form  of  ice  has  also  had  a  great  effect  in 


36  ELEMENTARY  SCIENCE 

determining  the  nature  of  the  topography,  especially  in 
northeastern    United    States.    Not    many    thousands    < 
years  ago  the  earth  appears  to  have  been  a  much  cooler 


FIG.  14. — Map  showing  how  a  great  glacier  once  covered  the  north- 
ern part  of  our  country. — After  CHAMULKLIN. 

place  than  it  is  now.  Picture  the  effect  this  would  have 
on  the  water.  Even  now  billions  of  tons  of  water  are 
"locked  up"  in  the  form  of  ice  in  the  polar  regions,  and  only 
a  relatively  small  amount  of  this  gets  into  circulation  each 


EFFECTS  OF  THE  MOVEMENTS  OF  WATER      37 

season  as  liquid.  But  in  those  colder  ancient  days  the 
accumulation  of  ice  in  polar  regions  was  much  greater. 
Slowly  a  great  sheet  of  ice  crept  southward  over  the  land, 
ironing  the  surface  as  it  went.  It  picked  up  huge  masses 
of  earth  and  boulders  and  carried  them  along.  Miles  deep 


FIG.  15. — Photograph  showing  a  terminal  moraine  of  coarse  boulders  deposited  at 
the  front  of  a  glacier. 

the  ice  accumulated  in  the  north,  and  it  flowed  to  the  south 
(see  Fig.  14).  The  southern  edge  of  the  great  ice-field 
changed  with  the  seasons,  and  changed  as  the  climate 
changed.  In  summer  it  melted  back  faster  than  it  flowed 
forward.  In  winter  it  regained  the  lost  ground.  Always, 
as  it  melted,  it  kept  depositing  ridges  and  sheets  of  rock 
and  soil.  Finally,  the  climate  changed  so  much  that  this 
ice-sheet  was  melted  far  back  to  the  north,  where  it  now 
remains,  covering  Greenland  and  other  regions.  But  the 


3g  ELEMENTARY  SCIENCE 

effects  which  it  produced  remain  to  this  day.  The  ridges 
and  sheets  of  rock  and  soil  the  ice  laid  down  are  with  us 
yet.  We  call  the  ridges  moraines  (see  Fig.  15),  and  we 
call  the  soil  the  ice  brought  down  glacial  till.  It  is  very 
fertile. 

Even  more  conspicuous  than  the  moraines  and  the  till 
are  the  effects  which  the  ice  produced  on  the  hills  and 


FIG.  16.— Drawing  showing  hills  of  which  the  rough  surfaces 
have  been  smoothed  off  by  a  glacier. 

valleys  they  travelled  over.  As  you  look  over  some  of  the 
beautiful  landscapes  of  central  New  York,  you  can,  in  the 
mind's  eye,  see  the  great  sheet  of  ice  again,  as  it  flowed 
with  tremendous  weight  down  over  the  rough  surface  of 
the  land,  and  then  retreated,  leaving  as  evidence  of  its 
visit  the  soft  and  graceful  curves  of  hill  and  valley  which 
now  delight  our  eyes  (see  Fig.  16).  It  left,  too,  many 
lovely  lakes.  The  basins  of  these  it  formed  by  damming 
up  great  valleys  with  the  material  it  dropped.  In  once 
firm-bedded  rocks  you  can  see  the  scratches  made  by 
material  in  the  ice  as  it  passed  over  them  (see  Fig.  17). 


EFFECTS  OF  THE  MOVEMENTS  OF  WATER      39 

In  many  other  regions  of  the  world,  even  in  regions  now 
tropical,  we  find  evidence  that  once  there  were  glaciers. 


FIG.  17. — Glacial  scratches. — After  SALISBURY. 

QUESTIONS 

1.  Why  did  people  once  prefer  to  live  near  rivers? 

2.  What  is  erosion? 

3.  Why  is  not  the  whole  surface  of  the  land  worn  down  nearly 

to  sea-level? 

4.  What  is  a  "drowned  valley"? 

5.  How  do  we  know  that  some  rocks  were  formed  in  the  sea? 

6.  What  becomes  of  the  rain  that  falls  on  the  earth? 

7.  Why  is  "ground- water"  important? 

8.  What  is  the  water-table? 

9.  In  what  layers  of  soil  does  the  ground-water  move? 

10.  Explain  an  artesian  well. 

11.  What  is  a  glacier,  and  what  does  it  do  to  the  surface  of  the 

earth? 


CHAPTER  V 
SOLUTION 

Lemonade  is  a  solution  of  sugar  and  lemon-juice  in 
water.  Sometimes  at  the  bottom  of  a  glass  of  lemonade 
you  have  found  some  sugar  that  has  not  dissolved.  The 
water  had  dissolved  all  the  sugar  it  could.  Such  a  solution 
is  called  a  saturated  solution.  Now  solutions  are  of  immense 
importance  to  life.  They  are  among  those  things  without 
which  life  could  not  possibly  continue.  We  must  under- 
stand them. 

The  word  solution  is  a  little  confusing  because  it  is  used 
in  two  ways.  It  is  the  name  of  a  process.  It  is  also  the 
name  of  a  thing.  The  process  of  dissolving  is  called  solu- 
tion, and  a  liquid  after  something  has  been  dissolved  in  it 
is  also  called  a  solution.  The  liquid  in  which  something 
has  been  dissolved  is  called  the  solvent,  and  the  substance 
dissolved  is  the  solute.  In  lemonade,  water  is  the  solvent, 
and  sugar  is  a  solute.  Substances  that  dissolve  are  called 
soluble;  if  they  do  not  dissolve  they  are  called  insoluble. 
Water  dissolves  more  substances  than  any  other  liquid, 
but  there  are  some  substances  that  are  not  soluble  in 
water  but  are  soluble  in  other  liquids.  We  rub  off  paint 
with  turpentine,  because  turpentine  will  dissolve  it  while 
water  will  not,  and  we  take  out  grass  stains  with  alcohol 
for  the  same  reason. 

There  are  many  substances,  like  sugar  and  salt,  that  dis- 
solve in  water.  This  means  that  when  they  are  put  into 
40 


SOLUTION  41 

water  the  invisible  particles  of  which  they  are  composed  begin 
to  spread  through  the  water  until  they  are  equally  distributed. 
This  is  the  law  of  solution.  Just  as  a  ball  thrown  up  in 
the  air  returns  to  earth  in  response  to  the  law  of  gravity, 
so  the  particles  of  a  solute  become  equally  distributed 
throughout  their  solvent,  in  response  to  the  law  of  solution. 
The  lemonade  in  a  glass  is  just  as  sweet  at  the  top  as  it  is 
at  the  bottom.  The  particles  of  sugar  are  equally  dis- 
tributed throughout  it. 

Now  let  us  see  why  solutions  are  so  important  in  life  as 
well  as  in  lemonade.  You  know  about  digestion.  You 
know  that  it  is  the  process  of  changing  food  from  solid 
to  liquid  form.  This  means  that  the  food  absorbed  by  our 
blood  is  a  solution.  Blood  itself  is  a  solution.  In  plant 
life,  solutions  are  just  as  important  as  in  animal  life.  All 
that  the  roots  take  in  from  the  soil  is  in  the  form  of  solu- 
tion, and  the  nourishing  sap  that  moves  inside  the  plant 
is  simply  water  with  various  substances  dissolved  in  it. 
And,  finally,  inside  the  bodies  of  plants  and  animals,  all 
the  processes  that  belong  to  life  itself  are  processes  that  occur 
in  solutions. 

You  must  distinguish  between  solution  and  suspension. 
When  we  wash  our  hands  we  may  say  that  the  soapy  water 
dissolves  the  dirt,  but  if  the  water  itself  turns  dirty,  that 
shows  that  we  have  a  case  of  suspension  and  not  of  solu- 
tion. Suspension  is  the  floating  in  a  liquid  of  fine  particles 
of  a  solid  or  of  another  liquid.  Streams  that  are  muddy 
in  flood  time  gradually  become  clear.  The  mud  suspended 
in  them  is  called  sediment  because  it  gradually  "settles." 
It  slowly  sinks  to  the  bottom.  But  the  solutes  in  solutions 
do  not  sink  to  the  bottom.  The  salt  in  sea-water  never 
settles.  It  becomes  a  solid  only  when  the  water  evaporates. 


42  ELEMENTARY  SCIENCE 

In  fresh  milk  the  cream  is  distributed  throughout  in  the 
form  of  tiny  droplets.  So  also  droplets  of  oil  are  distrib- 
uted in  salad  dressing.  These  are  examples  of  one  liquid 
in  suspension  in  another. 

In  lemonade  you  have  a  solid  (sugar)  and  a  liquid 
(lemon-juice)  dissolved  in  a  liquid  (water).  A  solution  may 
also  be  made  by  dissolving  a  gas  in  a  liquid,  and  this  proc- 
ess is  also  very  important  in  life.  It  has  something  to 
do  with  the  way  in  which  our  blood  absorbs  the  oxygen 
that  enters  our  lungs. 

A  very  important  case  of  gas  dissolved  in  a  liquid  is  the 
case  of  air  dissolved  in  water.  You  have  noticed  how  water 
may  enter  the  air  as  a  gas.  It  does  this  when  it  evapo- 
rates. Now  it  is  important  to  understand  that  the  reverse 
of  this  also  takes  place.  Air  enters  water.  It  is  dissolved 
by  water.  We  can  easily  prove  that  water,  as  found  in 
nature,  always  has  air  in  it.  Fishes  and  other  water 
animals  require  air  to  breathe,  and  their  source  of  air  is 
the  air  that  is  dissolved  in  water. 

If  you  gently  heat  water  in  a  beaker,  or  in  a  porcelain- 
lined  vessel,  the  first  change  you  will  note  will  be  the  for- 
mation of  hundreds  of  tiny  bubbles  on  the  bottom  and 
sides  of  the  vessel.  These  are  very  different  from  the 
large  bubbles  of  steam  that  form  later,  when  the  water 
has  reached  the  boiling-point.  These  tiny  bubbles  appear 
long  before  steam  arises  from  the  surface.  They  appear 
when  the  water  is  only  a  few  degrees  warmer  than  the 
surrounding  air.  Then  if  you  stop  heating  the  water,  and 
set  it  aside  without  shaking,  these  bubbles  remain  for  a 
time,  even  after  the  water  has  cooled  down  to  its  ordinary 
temperature.  Of  course  they  would  not  do  this  if  they 
were  bubbles  of  steam.  Steam  bubbles  would  quickly 


SOLUTION  43 

change  to  liquid  again  as  the  temperature  fell.  These 
tiny  bubbles  are  bubbles  of  air. 

If  you  have  ever  fished  with  minnows  for  bait,  you  know 
that  you  must  change  the  water  frequently  to  keep  the 
minnows  from  dying.  But  if  you  take  a  pail  of  minnows 
away  from  the  stream,  and  cannot  change  the  water  fre- 
quently, they  will  live  much  longer  if  you  stir  the  water 
frequently.  Can  you  explain  this? 

Air  is  not  the  only  gas  that  is  readily  absorbed  by  water. 
Some  gases  dissolve  hi  water  to  a  remarkable  extent.  Thus 
water  can  dissolve  one  thousand  times  its  bulk  of  the  gas 
ammonia.  A  gas  that  is  present  in  air,  and  is  very  impor- 
tant in  the  lives  of  both  plants  and  animals,  is  carbon 
dioxide.  This  gas  is  given  off  from  our  lungs  when  we 
exhale.  It  is  also  one  of  the  products  of  burning.  It  is 
used  by  plants  in  the  manufacture  of  food.  It  is  readily 
absorbed  by  water,  and  all  natural  waters  contain  some 
of  it.  Sea-water  contains  more  carbon  dioxide  than  does 
the  air  above  it.  Soda-water  is  nothing  but  water  charged 
with  carbon  dioxide.  The  water  is  placed  in  a  metal  con- 
tainer, and  the  gas  forcea  into  it  under  high  pressure.  The 
container  is  then  firmly  sealed.  As  soon  as  any  of  this 
water  is  drawn  off  it  begins  to  fizz  and  bubble  the  way 
you  have  seen  it  in  a  soda-water  glass.  This  is  because 
the  pressure  is  removed,  and  the  extra  load  of  gas  is  rush- 
ing to  escape. 

We  have  been  calling  air  a  gas.  It  is  time  to  explain 
that  it  is  a  mixture  of  gases.  Four-fifths  of  it  is  a  gas 
called  nitrogen,  and  nearly  all  the  rest  of  it  is  the  gas  called 
oxygen.  Less  than  half  of  one  per  cent  of  it  is  carbon 
dioxide,  yet  that  one-half  of  one  per  cent  is  absolutely 
necessary  to  the  making  of  our  food.  All  these  gases,  as 


44  ELEMENTARY  SCIENCE 

well  as  the  water  vapor  in  the  air,  are  perfectly  trans- 
parent, and  are  evenly  distributed.  Just  as  the  substances 
that  make  up  a  solution  tend  to  become  equally  distributed, 
so  gases  that  come  together  tend  to  become  equally  dis- 
tributed. The  process  by  which  one  gas  spreads  itself 
evenly  through  another  is  called  the  process  of  diffusion. 

Water,  as  we  find  it  in  nature,  is  never  perfectly  pure. 
That  is,  it  always  contains  particles  of  other  substances 
along  with  the  particles  of  water.  However,  these  natural 
impurities  do  not  discolor  the  water,  and  may  not  make  it 
less  fit  to  drink.  In  fact,  the  most  agreeable  drinking  water 
does  contain  impurities,  while  perfectly  pure  water  tastes 
flat  and  insipid.  Any  one  who  drinks  distilled  water, 
having  been  in  the  habit  of  drinking  well-water,  finds  it 
disagreeable  at  first.  Springs  that  are  famous  for  the 
health-giving  value  of  their  water  owe  this  to  the  presence 
in  solution  of  certain  minerals  or  gases,  or  both. 

Hard  and  Soft  Water.  — The  difference  between  hard 
and  soft  water  is  that  the  former  contains  more  of  dissolved 
matter  than  the  latter.  There  are,  of  course,  all  sorts  of 
degrees  of  "hardness"  and  "softness"  of  water,  depending 
upon  the  varying  amounts  of  matter  dissolved  in  them. 

Spring-water  is  usually  regarded  as  the  purest  for  drink- 
ing purposes,  because  it  has  been  thoroughly  filtered  by 
soaking  through  the  soil.  This  process  removes  particles 
which  are  suspended  in  the  water  (as  particles  of  mud), 
and  so  renders  the  water  quite  clear,  but  it  has  no  effect 
on  the  material  which  is  dissolved  in  the  water.  In  fact, 
this  process  of  soaking  through  the  soil  is  the  very  process 
by  which  water  becomes  hard.  It  dissolves  certain  soil  ma- 
terials as  it  goes  along  and  carries  them  with  it  in  solution. 


SOLUTION  45 

Hard  water  is  not  good  for  washing.  When  you  use 
soap  with  hard  water  a  dirty  sediment  is  produced.  This 
prevents  the  formation  of  a  lather,  unless  a  good  deal  of 
extra  soap  is  used.  When  hard  water  is  boiled,  some  of  the 
mineral  matter  in  it  is  deposited  on  the  sides  and  bottom  of 
the  boiler  or  kettle.  You  may  have  noticed  this  formation 
(fur}  on  the  inside  of  a  teakettle.  It  sometimes  forms  what 
is  known  as  boiler-scale  on  the  inside  of  the  pipes  in  steam- 
boilers,  and  renders  them  useless  until  they  have  been 
cleaned,  which  is  an  expensive  process. 

River-water  is  usually  soft,  though  it  contains  some  min- 
eral matter,  and  may  carry  a  good  deal  of  soil  in  suspension, 
thus  giving  it  a  muddy  appearance.  Lake-water  is  softer 
than  well  or  spring  water,  but  usually  harder  than  river- 
water.  Rain-water  is  very  soft,  and  hence  is  collected 
in  cisterns  for  use  in  washing.  Though  best  for  washing, 
it  is  not  best  for  drinking.  It  contains  impurities  washed 
from  the  atmosphere  and  from  roofs,  and  its  taste  is  less 
agreeable  than  that  of  other  kinds  of  water. 

QUESTIONS 

1.  What  is  a  solution? 

2.  Why  do  we  use  paint? 

3.  Do  fishes  breathe  air?     Explain. 

4.  Why  do  bubbles  form  in  heated  water  before  it  is  hot  enough 

to  boil?     How  could  you  prove  that  these  were  not  steam 
bubbles? 

5.  What  is  soda-water,  and  why  does  it  fizz  ? 

6.  What  is  the  composition  of  the  air? 

7.  What  is  the  difference  between  hard  and  soft  water? 

8.  Why  does  "fur"  form  on  the  inside  of  a  teakettle? 


CHAPTER  VI 

MAN'S  SUPPLY  OF  WATER.    THE  INSIDE  OF 
THE  EARTH 

The  Sources  of  Water-Supply.  —  Where  does  the  water 
that  you  use  in  your  own  house  come  from?  Probably 
all  you  have  to  do  to  get  it  is  to  turn  on  a  tap  and  let  it 
run.  Think  now  where  it  comes  from  and  how  it  gets  to 
you. 

Lakes,  rivers,  springs,  wells,  and  cisterns  are  the  sources 
from  which  men  get  the  water  they  need.  Nearly  all  great 
cities  were  originally  located  on  lakes  or  rivers;  these  were 
important  for  water-supply  as  well  as  for  transportation. 
But  when  homes  had  to  be  made  in  the  country,  away 
from  lakes  and  rivers,  springs  were  looked  for  and  wells 
were  dug.  No  house  was  built  unless  there  was  near  it 
a  convenient  supply  of  water.  Wells  are  very  ancient. 
The  Old  Testament  and  other  books  of  ancient  history  tell 
of  their  important  part  in  the  life  of  the  people. 

A  well  is  just  a  hole  dug  in  the  ground  with  water  in  it. 
A  deepened  spring  is  a  sort  of  well.  We  may  say  that  the 
reason  a  well  has  water  in  it  is  because  the  hole  has  tapped 
an  underground  spring.  So  there  is  no  absolute  difference 
between  a  well  and  a  spring.  There  is  water  in  the  ground. 
When  this  water  comes  to  the  surface  of  its  own  accord, 
we  have  a  spring  (see  Fig.  1 1) ;  when  we  dig  for  it,  we  have 
wells.  Whether  we  strike  water  or  not  when  we  dig  de- 
pends a  good  deal  on  chance,  but  not  so  much  as  it  used  to. 
Modern  science  furnishes  information  which  enables  ex- 
46 


INSIDE  OF  THE   EARTH  47 

perts  to  tell  with  a  good  deal  of  certainty  what  to  expect  in 
the  way  of  well-water.  But  in  the  old  days,  when  a  well 
was  to  be  located,  it  was  quite  customary  to  employ  a 
"water- witch."  A  water-witch  was  a  person  supposed  to 
have  mysterious  power  to  locate  favorable  spots  for  wells. 
This  was  done  with  the  aid  of  "divining-rods."  A  favorite 
form  of  such  rod  was  a  forked  branch  of  hazel,  or  peach. 
The  rod  was  held  by  the  forks,  the  main  stem  pointing 
straight  forward.  Then,  as  the  holder  passed  over  water, 
the  main  stem  was  supposed  to  twitch  downward.  It 
has  never  been  possible  to  prove  that  this  is  so,  and  it 
seems  absurd,  yet  there  are  many  people  to-day  who 
still  believe  there  is  "something  in  it."  In  support  of 
their  belief,  they  point  to  wells  which  have  been  located 
in  that  way.  Does  this  prove  anything  ?  Yet  many  things 
which  people  believe  to  be  true  are  founded  on  no  better 
evidence  than  this.  The  great  work  of  science  is  to  find 
out  what  is  true  and  what  is  false. 

The  Inside  of  the  Earth.  —  We  cannot  think  of  under- 
ground water  in  wells  and  springs  and  hidden  streams 
without  thinking  of  the  inside  of  the  earth.  Have  you 
not  wondered  about  this  mysterious  region?  Have  you 
not  wanted  to  explore  caves,  and  to  see  the  stalactites  like 
rock  icicles  (see  Fig.  18)  whose  pictures  you  have  seen  in 
geographies?  Scientists,  too,  have  done  a  good  deal  of 
wondering  about  the  inside  of  the  earth,  but  they  have  not 
found  out  very  much.  The  deepest  mines  are  but  mere 
scratchings  when  you  think  of  all  that  is  beneath  them. 
It  is  more  than  eight  thousand  miles  through  to  China, 
and  the  deepest  borings  of  man  go  down  only  a  few  thousand 
feet.  And  yet  these  borings,  and  other  things,  have  given 


48 


ELEMENTARY  SCIENCE 


a  good  deal  to  go  on,  so  that  we  feel  fairly  certain  of  the 
following  things: 

i.  That  the  earth,  as  a  whole,  is  solid,  and  the  deeper 
you  go  the  more  solid  it  gets.    The  pressure  increases  and 


FIG.  1 8.— Photograph  of  limestone  cave,  showing  rock 
icicles  (stalactites). 

becomes  so  tremendous  that  the  real  inside  of  the  earth  is 
as  firm  as  steel. 

2.  Yet  some  underground  regions  are  fluid.     They  are 
also  very  hot.    Volcanoes  prove  this  (see  Fig.  !9).     The 


INSIDE   OF  THE   EARTH 


49 


"molten  lava"  they  throw  out  when  they  are  in  eruption 
comes  from  the  depths  of  the  earth.  So  it  seems  that,  as 
you  go  down,  the  heat  increases  as  well  as  the  pressure. 
Judged  by  conditions  in  deep  mines,  the  thermometer  goes 
up  at  the  rate  of  about  one  degree  for  every  hundred  feet 
of  depth.  This  varies  a  great  deal  with  different  localities, 
and  for  the  first  hundred  feet  or  so  it  usually  does  not 

apply  at  all.     In      . , 

fact,  cellars  and 
caves  are  usually 
much  cooler  than 
the  air  above. 
Why  is  this  so? 

3.  The  earth 
was  once  very  hot; 
so  hot  that  noth- 
ing at  the  surface 
was  solid.  It  was 
all  either  liquid  or 
gas.  The  cooling- 
off  process  and  the 
formation  of  solid  rocks  probably  required  millions  of 
years.  The  changes  that  have  occurred  to  the  land  areas 
since  they  were  solidified  have  been  produced  chiefly  by 
running  water,  and  by  movements  of  the  surface  that 
have  changed  sea  to  dry  land  and  dry  land  back  to  sea 
again.  To-day  we  find  far  inland,  and  often  deep  under 
the  surface,  sure  evidence  that  the  sea  was  once  there. 
We  find  accumulations  of  sea-shells',  and  formations  of 
limestone  that  were  undoubtedly  made  from  sea-shells 
(see  Fig.  9).  We  also  find  under  the  surface  deposits  of 
coal,  and  coal  is  a  formation  due  to  the  accumulation  of  the 


FIG.  ig. — Volcano. 


5o  ELEMENTARY  SCIENCE 

bodies  of  plants,  probably  in  swamps.  These  accumula- 
tions subsequently  underwent  changes  which  were  largely 
due  to  pressure.  Hard  coal  (anthracite}  has  been  sub- 
jected to  greater  pressure  than  soft  coal  (bituminous). 

4.  The  outer  part  of  the  earth  is  largely  composed  of 
layers  called  strata  (singular,  stratum}.  Its  stratified  nature 
may  be  observed  in  many  mountain  regions  where  the 
upfoldings  are  sharp  and  broken.  Sometimes  it  may  be 
seen  on  the  sides  of  railway  cuttings,  and  always,  when 
deep  wells  are  dug,  the  boring  passes  through  different 
layers,  as  of  clay,  gravel,  and  rock,*  of  which  there  are 
many  varieties. 

The  early  geologists  were  much  puzzled  by  these  strata 
and  brought  forward  many  interesting  theories  as  to  their 
cause.  But  as  knowledge  grew,  it  became  easy  to  under- 
stand that  they  are  due  simply  to  the  surface  movements 
and  water  work  that  you  have  been  considering.  For 
example,  a  layer  of  deep  mud  might  have  formed  over  the 
bottom  of  a  shallow  sea,  not  far  from  the  land.  Then 
by  a  shift  of  the  surface  (a  slow  and  gentle  earthquake) 
this  region  may  have  become  deep  sea,  far  from  land. 
The  wash  of  the  land  may  have  reached  it  no  more,  but 
the  shells  of  tiny  marine  animals,  for  long  ages,  accumu- 
lated on  top  of  the  mud.  Then,  due  to  another  shift, 
came  a  period  in  which  sand  was  the  chief  thing  laid  down. 
So  these  changes  went  on.  Due  to  the  pressure  of  new 
layers,  and  to  chemical  and  other  physical  changes,  the 
older  layers  became  hardened  into  rock.  Then  at  last 
there  may  have  come  a  great  disturbance,  due  to  an  adjust- 

Rock  means  any  hard  part  of  the  earth's  crust.  It  does  not  need  to 
be  broken.  When  you  say  you  pick  up  a  rock,  it  would  be  more  accurate 
to  say  a  piece  of  rock, 


INSIDE   OF   THE   EARTH  51 

ment  of  differences  between  the  forces  of  heat  and  of 
pressure.  As  a  result,  these  layers,  and  others,  may  have 
been  forced  up,  pressed  together,  broken  and  distorted, 
and  the  result  was  a  new  range  of  mountains  (see  Fig.  7). 
And  in  such  mountains  to-day  we  may  see  what  was  once 
mud  changed  to  a  kind  of  rock  called  shale,  what  was  once 


FIG.  20. — Igneous  rock;  a  granite  quarry. 

a  layer  of  shells  changed  to  limestone,  and  what  was  once 
sand  changed  to  sandstone. 

5.  As  to  their  origin,  all  rock  or  formations  of  rock  may 
be  classed  as  igneous  or  sedimentary.  Igneous  means 
related  to  fire  (ignis,  fire),  and  igneous  rocks  are  those 
whose  substance  was  once  molten.  They  have  come 
to  their  present  state  through  cooling,  pressure  also  usually 
having  a  good  deal  to  do  with  it.  Granite  is  an  example 
of  igneous  rock  (see  Fig.  20).  Sedimentary  rock  is  rock 


52  ELEMENTARY  SCIENCE 

whose  material  once  was  sediment;  that  is,  it  was  laid 
down  in  water.  Limestone  and  sandstone  are  sedimentary 
rocks  (see  Fig.  7). 

6.  The   strata  or  formations  of  the  earth's  crust  are 
very  different  with  respect  to  their  relations  to  water. 
Some  are  much  more  soluble  in  water  than  others.    Lime- 
stone is  especially  soluble  in  water.     Sandstone  is  com- 
posed of  grains  of  sand  held  firmly  together  by  a  sort  of 
cement.    Water  may  act  on  the  cement  and  leave  loose 
sand.    Large  caves  occur  in  limestone  regions  and  are  due 
to  underground  water  having  dissolved  and  carried  away 
particles  of  the  rock.    The  stalactites  are  due  to  dripping 
water  which,  at  the  point  of  drip,  deposited  certain  solutes 
they  had  been  carrying  (see  Fig.  18). 

7.  Clay,  gravel,  and  sand,  and  mixtures  of  these  with 
each  other  and  with  organic  substances,  form  what  we 
call  50*7  and  subsoil.    They  are  the  latest  geological  forma- 
tions.   Gravel  and  sand,  being  loose  formations,  can  hold 
a  good  deal  of  water.     Clay,  whose  particles  are  very 
small,  packs  down  into  a  firm  formation  which  water  pene- 
trates very  slowly.    Some  of  the  best  wells  are  made  by 
boring  into  formations  of  wet  sand  or  gravel  down  below 
layers  of  clay.    The  water  may  have  entered  the  strata 
of  sand  or  gravel  many  miles  away  where  they  outcrop  at 
the  surface  (see  Fig.  u).    It  may  have  soaked  through 
them,  being  hindered  in  working  upward  by  the  dense 
layers  of  clay  above.     So,  in  northern  Illinois,  many  of  the 
best  wells  tap  a  layer  of  sand  whose  outcrop  is  far  away  in 
Wisconsin.     Springs  are  caused  by   the  outcropping  of 
layers  which  have  received  water  somewhere  else.     A  rtesian 
wells  (so  called  because  first  found  in  Artois,  France)  are 
those  whose  water  rises  to  the  surface  without  pumping, 


INSIDE   OF  THE   EARTH 


53 


sometimes  with  sufficient  force  to  form  a  natural  fountain. 
This  is  because  the  well-boring  has  tapped  water  which  is 
under  pressure,  due  to  its  intake  being  at  a  greater  altitude 
than  the  mouth  of  the  well  (see  Figs.  12  and  13).  Or,  in 
some  cases,  the  pressure  is  due  to  the  confinement  of  the 
liquid  between  two  deep-placed  layers  which  it  cannot 


FIG.  2i.— Oil-wells. 

penetrate.  This  is  the  explanation  of  gushing  oil-wells 
(see  Fig.  21).  When  formations  of  oil  and  natural  gas  are 
tapped,  the  gas  naturally  comes  to  the  surface  first.  Gas 
and  oil  are  found  in  the  earth  both  together  and  separately. 
Both  are  believed  to  have  an  organic  origin,  that  is,  they 
have  resulted  from  extensive  chemical  and  physical  changes 
in  accumulations  of  organic  substances.  By  organic  sub- 
stances we  mean  those  derived  from  the  bodies  of  living 
things  (plants  and  animals);  all  other  substances  are 
inorganic. 


54  ELEMENTARY  SCIENCE 

This  chapter  began  to  tell  of  man's  supply  of  water,  and 
then,  to  explain  about  wells  and  springs,  it  went  clear  back 
to  the  start  of  things.  It  told  a  little  of  the  history  of  the 
earth.  This  may  seem  like  getting  away  from  the  story. 
But  you  will  find  it  is  always  so  in  studying  the  story  of 
nature.  In  order  really  to  understand  the  little  things, 
you  have  to  go  back  to  the  big  and  ancient  things.  It  is 
a  continuous  story,  but  it  continues  out  and  back  in  every 
direction  rather  than  in  a  straight  line.  To  understand 
what  we  see  to-day,  we  must  understand  these  great  forces 
which  have  shaped  the  earth  since  it  began,  and  which  still 
govern  it  and  its  tiny  creatures. 

QUESTIONS 

1.  Why  does  a  well  have  water  in  it? 

2.  How  was  coal  made? 

3.  What  is  meant  by  "sedimentary"  rock? 

4.  Describe  an  ordinary  well,  a  spring,  an  artesian  well,  and  a 

gushing  oil-well. 


CHAPTER  VII 
THE  SUCTION-PUMP.    ATMOSPHERIC  PRESSURE 

You  have  noted  that  man  discovered  a  very  long  time 
ago  that  a  good  way  to  get  pure  drinking  water  is  to  dig 
for  it.  But  having  dug  for  it  and  found  it,  he  had  a  good 
deal  of  trouble  in  getting  it  out  of  the  hole.  The  ancient 
way  was  to  lower  a  pail,  and 
then  to  pull  it  up.  There  was 
considerable  danger  of  falling  in 
while  you  were  doing  this.  So 
they  built  curbs  about  the  wells. 
Then  some  bright  person  dis- 
covered that  you  could  pull  the 
pail  up  more  easily  by  winding 

...  .  .  f  FIG.  22.— The  windlass. 

it  up  on  a  revolving  bar  which 

you  turned  with  a  handle.  This  was  the  invention  of  the 
windlass  (see  Fig.  22).  But  it  was  a  long  time  after  wind- 
lasses were  invented  before  any  one  happened  to  get  the 
idea  of  a  pump. 

To  understand  how  a  pump  works  you  must  understand 
atmospheric  pressure.  Atmosphere  is  simply  another  word 
for  air.  You  may  have  heard  that  air  at  sea-level  exerts 
a  pressure  of  fifteen  pounds  to  the  square  inch.  This  at- 
mospheric pressure  is  due  to  the  fact  that  this  mixture  of 
gases  we  call  air  extends  for  some  miles  upward  from  the 
earth,  and  that  this  great  sea  of  air,  at  the  bottom  of  which 
we  live,  has  a  certain  weight.  It  is  so  evenly  distributed 
about  us,  and  w\.  are  so  used  to  it,  that  we  are  not  con- 

55 


5  6  ELEMENTARY  SCIENCE 

scious  of  it  at  all,  any  more  than  fish  are  conscious  of  the 
weight  of  water  in  which  they  swim.  Though  we  are 
not  conscious  of  this  weight  of  air  through  our  sensations, 
yet  it  is  a  very  real  thing,  and  it  does  a  great  deal  of  work 
which  is  for  our  advantage. 

You  take  advantage  of  air-pressure  every  time  you 
suck  lemonade  through  a  straw.  As  the  straw  stands  free 
in  the  glass,  the  air-pressure  within  it  and  the  air-pressure 
outside  of  it  are  equal,  and  the  lemonade  stands  at  the 
same  level  within  and  without  the  straw.  But  the  instant 
you  put  your  lips  to  the  straw  and  draw  into  your  mouth 
the  air  which  is  in  it,  what  happens?  The  lemonade 
immediately  follows  the  air  up  to  your  lips.  Why  does 
it  do  this?  It  is  because  you  have  removed  the  air  which 
was  holding  it  down.  The  air  outside  is  pressing  down  on 
the  surface  of  the  lemonade,  and  it  is  this  pressure  which 
makes  the  lemonade  go  up  the  straw.  Suppose  there  was 
an  air-proof  cover  over  the  glass  except  where  the  straw 
protruded.  Could  you  still  suck  up  the  lemonade?  Try 
it  and  see.  Have  you  ever  tried  to  pour  liquid  out  of 
a  can  with  only  one  hole  in  it?  Why  is  it  necessary  to 
have  two  holes  in  order  that  the  liquid  may  flow  readily 
out  of  one  of  them  ? 

Now  it  is  this  very  principle  which  operates  when  a 
pump  works.  The  surprising  thing  is  that  it  was  so  long 
before  any  one  thought  of  taking  advantage  of  it  in  this 
way.  But  we  should  remember  that  things  which  seem 
simple  and  easy  to  us  now  were  not  at  all  simple  and  easy 
for  our  ancestors.  If  we  had  been  in  their  place  we  should 
have  done  no  better  than  they,  for  they  were  quite  as 
bright  as  we.  But  we  have  the  great  advantage  that  we 
may  learn  quickly  and  easily  what  mankind  learned  very 


THE  SUCTION-PUMP 


57 


slowly  and  with  great  difficulty.  For  this  we  should  be 
truly  grateful.  We  should  accept  our  wonderful  "heritage 
of  knowledge"  with  an  understanding  of  all  the  toil  and 
all  the  time  it  took  to  build  it  up.  For  what  we  know 
there  is  little  credit  to  ourselves.  About  all  we  have  to  do 
is  to  open  a  book  and  read. 

The  most  important  thing  about  a  pump  is  the  piston 
(see  Fig.  23).  It  works  up  and  down  in  a  cylinder.  When 
you  push  the  handle  of  the  pump  down, 
the  piston  comes  up  in  the  cylinder,  and 
does  the  same  thing  that  you  do  when 
you  suck  a  straw.  //  removes  the  pressure 
on  the  water  just  below  it.  The  pressure 
of  air  on  the  water  in  the  well  forces  the 
water  in  the  pipe  to  follow  the  piston  up 
the  cylinder.  Of  course  the  piston  must 
be  air-tight,  and  there  must  be  a  valve  hi 
it  which  will  let  the  water  come  through 
but  will  not  let  it  get  back.  So  when 
you  force  the  piston  down  again,  the 
water  which  has  entered  the  cylinder 
will  pass  through  it  (the  piston)  and  be  in  a  position  to 
run  out  of  the  spout.  Also  there  must  be  a  valve  at  the 
point  where  the  pipe  joins  the  cylinder.  Why?  All  this 
you  can  understand  better  by  studying  the  figure  and  ex- 
plaining it.  When  a  pump  has  a  leaky  valve,  you  can 
sometimes  make  it  work  by  priming  it,  that  is,  by  pouring 
in  water  at  the  top.  Can  you  explain  this? 

If  the  distance  between  the  surface  of  the  water  in  the 
well  and  the  piston  in  such  a  pump  as  we  have  been  describ- 
ing is  greater  than  about  thirty-two  feet,  the  pump  will 
not  work.  This  should  not  surprise  you  when  you  remem- 


Fic.  23.— The  suc- 
tion-pump; try  to 
explain  the  differ- 
ent parts. 


58  ELEMENTARY  SCIENCE 

her  that  atmospheric  pressure  is  not  an  unlimited  thing. 
It  is  limited,  as  you  have  noted,  to  fifteen  pounds  to  the 
square  inch.  Of  course  it  will  not  force  a  column  of  water 
up  an  indefinite  distance.  About  thirty-two  feet  is  the  limit. 
Atmospheric  pressure  is  well  illustrated  by  the  siphon 
(see  Fig.  24).  The  siphon  has  been  used  since  ancient 
times  for  drawing  liquid  from  a  higher  level  to  a  lower  level 
over  the  side  of  the  higher  vessel.  The  tube  must  be 
filled  with  the  liquid  in  order  to  start  the  process.  As 
long  as  the  distance  from  a  to  d  is  less  than  the  height 
at  which  atmospheric  pressure  will  hold 
a  column  of  water,  and  as  long  as  the 
tube  remains  filled  with  water,  it  will 
continue  to  flow  from  the  bottom  of 
the  tube.  For  as  fast  as  it  flows  down 
and  away,  it  is  replaced  by  water 
rising  to  the  top  of  the  arch  on  account 

FlG'  of4'theTsliphonndple     °*  the  atmosPneric  pressure. 

What  effect  should  you  expect  atmos- 
pheric pressure  to  have  upon  the  boiling-point  of  water  ?  Will 
it  take  less  heat  or  more  heat  to  make  water  boil  at  the  top 
of  a  mountain?  Amateur  campers  in  the  mountains  have 
found  that  the  water  may  all  boil  away  without  cooking  the 
beans,  or  whatever  it  is  they  are  trying  to  boil.  Why  is  this  ? 

Measuring  Atmospheric  Pressure.  —  An  aneroid  ba- 
rometer is  an  instrument  much  used  in  mountain  travel.  It 
enables  you  to  estimate  how  high  up  you  are,  and  it  does 
this  by  recording  the  atmospheric  pressure.  There  is  a 
little  metal  box  with  a  vacuum  inside,  that  is,  a  space  from- 
which  the  air  has  been  withdrawn.  The  pressure  of  the 
air  outside  causes  imperceptible  movements  of  the  metal 


THE  SUCTION-PUMP  59 

sides  of  this  box.  This  is  because  there  is  no  pressure 
within  the  box.  These  imperceptible  movements  are  mag- 
nified by  a  contrivance  which  records  them  by  movements 
of  a  hand  on  a  dial.  Since  certain  pressures  correspond  to 
certain  altitudes,  the  hand  and  dial  show  the  altitude  as 
well  as  the  pressure  (see  Fig.  25).  However,  since  air- 
pressure  varies  with  local  conditions,  as  in  storms,  you  can 
see  that  an  aneroid  record  of  altitude  is  not  always  exact. 
The  word  aneroid  means  without  fluid.  Fluid  barometers 
are  also  used.  A  mercurial  barom- 
eter is  one  in  which  changes  of  pres- 
sure are  recorded  by  the  rise  or  fall 
of  a  column  of  mercury.  As  you 
already  know,  the  rise  and  fall  of  a 
column  of  mercury,  in  the  instru- 
ment we  call  the  thermometer,  re- 
cords changes  of  heat.  But  note 
this  important  difference.  In  a 
thermometer  the  mercury  is  in  a  FIG.  25.— The  aneroid  barom- 
closed  tube  and  is  not  sensitive  to 

changes  in  pressure.  But  in  a  mercurial  barometer  the 
mercury  must  be  exposed  at  some  point  to  the  air  so  that 
it  will  be  sensitive  to  changes  in  pressure;  also  there  must 
be  sufficient  room  for  expansion  of  the  mercury  outside 
the  tube  so  that  changes  in  its  volume  due  to  heat  will  not 
seriously  affect  the  reading  of  changes  due  to  pressure 
(see  Fig.  26). 

Standard  atmospheric  pressure  is  the  average  pressure 
at  sea-level.  You  have  noted  that  this  is  a  pressure  of 
fifteen  pounds  to  the  square  inch.  This  was  determined 
by  measuring  at  sea- level  the  height  of  a  column  of  mer- 
cury held  up  in  a  mercurial  barometer;  this  height  is  about 


60  ELEMENTARY  SCIENCE 

thirty  inches.  Since  mercury  weighs  a  half-pound  to  the 
cubic  inch,  it  follows  that  the  standard  atmospheric  pres- 
sure amounts  to  fifteen  pounds  to  the  square  inch.  Simi- 
larly, in  barometric  readings,  thirty  inches  is  taken  for  the 
standard.  Pressure  which  the  barometer  records 
1  f*  by  standing  at  over  thirty  inches  is  said  to  be 
high;  pressures  recorded  at  less  than  thirty 
inches  are  said  to  be  low.  You  already  know 
that  a  barometer  is  used  to  foretell  weather,  and 
weather  is  largely  due  to  states  of  atmospheric 
pressure.  If  the  conditions  of  atmospheric  pres- 
sure over  a  large  area  are  known,  the  weather 
about  to  follow  can  be  foretold  with  a  good  deal 
of  certainty. 

The  standard  of  atmospheric  pressure  (fifteen 
pounds  to  the  square  inch)  is  also  used  as  a 
standard  in  measuring  other  kinds  of  pressures; 
it  is  referred  to  as  a  pressure  of  one  atmosphere. 
Thus  the  pressure  of  water  may  be  measured  in 
atmospheres.  Water  at  thirty  feet  below  its 
surface  has  a  pressure  of  one  atmosphere,  etc. 
Evidently  this  is  a  matter  of  importance  in 
under-water  constructions,  as  in  building  of  sub- 
marine  boats.  Careful  allowance  must  be  made 
*or  t^le  Pressures  which  are  to  be  withstood. 

Atmospheric  pressure  was  first  measured  by  a 
famous  experiment  known  as  TorricellVs  (tor-ri-chelly)  ex- 
periment. TorricelH  (1608-47)  was  a  pupil  of  Galileo,  the 
famous  Italian  scholar  of  whom  you  have  already  read. 
Galileo  himself  noted  that  suction-pumps  were  unable  to 
lift  water  more  than  about  thirty-two  feet,  and  he  probably 
suspected  that  this  rise  was  due  to  atmospheric  pressure. 


THE  SUCTION-PUMP 


61 


You  see  pumps  were  used  long  before  they  were  explained. 
It  remained  for  Torricelli  to  explain  them,  and  also  to  dem- 
onstrate the  limits  of  atmospheric  pressure.  He  did  this 
by  means  of  an  experiment  with  mercury,  which,  being  a 
much  heavier  liquid  than  water,  does  not  require  so  long 
a  tube  to  measure  the  atmospheric  pressure  against  it. 
This  experiment  may  be  repeated  as  follows  (Fig.  27):  Fill 
with  mercury  a  strong  glass  tube  closed 
at  one  end.  The  tube  should  be  about 
thirty-two  inches  long.  Invert  the 
tube,  closing  the  open  end  with  a  finger 
(A).  Immerse  this  end  in  a  dish  of  mer- 
cury and  then  remove  the  closing  finger. 
The  mercury  then  falls,  leaving  a  vac- 
uum at  the  top  (B).  But  if  the  experi- 
ment is  well  performed,  the  mercury 
will  fall  only  a  short  distance.  It  will 
stop  at  about  thirty  inches  above  the 
surface  of  the  mercury  in  the  dish. 
This  thirty-inch  column  of  mercury  is, 
then,  a  measure  (in  mercury)  of  at- 
mospheric pressure.  Thus  you  see  that 
the  Torricelli  experiment  is  actually  the  construction  of  a 
mercurial  barometer. 

Pascal,  a  great  French  mathematician  (1623-62),  rea- 
soned that  if  the  mercury  in  the  Torricelli  experiment  is 
really  supported  by  atmospheric  pressure,  then  the  column 
should  become  shorter  at  a  higher  altitude.  So  he  per- 
formed the  experiment  at  the  top  of  a  high  tower  in  Paris 
and  found  a  decrease  in  the  column  which  corresponded 
to  his  computations.  Soon  afterward  this  principle  was 
confirmed  by  performing  the  experiment  on  a  mountain. 


FIG.  27. — Diagram  illus- 
trating Tonicelli's  ex- 
periment. 


62  ELEMENTARY  SCIENCE 

In  this  way  it  was  found  that  the  height  of  a  column  of 
mercury  could  be  used  as  a  measure  of  altitude  above  sea- 
level. 

QUESTIONS 

1.  Explain  what  happens  when  you  suck  up  lemonade  through  a 

straw. 

2.  Why  can't  you  pour  liquid  out  of  a  can  with  only  one  hole  in 

it? 

3.  Explain  how  a  pump  works. 

4.  Explain  how  a  siphon  works. 

5.  Explain  how  an  aneroid  barometer  works.     A  mercury  barom- 

eter. 

6.  Why  is  the  barometer  important  for  the  "weather-man"? 


CHAPTER  VIII 

ARCHIMEDES'  PRINCIPLE.     WATER-PRESSURE 
AND  WATER-WORKS 

Archimedes  (287-212  B.C.)  was  the  greatest  of  the 
ancient  mathematicians.  He  lived  in  Syracuse  on  the 
island  of  Sicily.  The  king  of  Syracuse  in  Archimedes' 
time  once  ordered  of  a  goldsmith  a  crown  of  pure  gold. 
It  was  delivered  to  him.  It  looked  like  pure  gold  and  it 
had  the  proper  weight.  Still  the  king  had  reason  to  sus- 
pect that  the  goldsmith  had  cheated  by  melting  up  silver 
along  with  the  gold.  He  asked  Archimedes  to  discover 
a  method  of  proving  the  fraud.  A  modern  chemist  could 
have  solved  the  problem  in  a  minute,  but  chemistry  in 
those  days  was  unknown.  So  the  mathematician  put 
his  wits  to  work.  He  was  sorely  puzzled.  One  day,  while 
in  the  public  bath,  he  noticed  that  the  part  of  his  body 
under  the  water  was  much  lighter  (more  buoyant)  than  the 
part  above  it.  He  got  to  thinking  about  this.  Suddenly 
he  jumped  from  the  bath  like  a  crazy  man  (so  the  story 
goes),  rushed  into  the  street  without  waiting  to  dress,  and 
ran  home  shouting  "Eureka!  Eureka!"  which  means  "I 
have  found  it." 

'  Now  what  had  occurred  to  Archimedes  was  this:  He 
knew  that  gold  and  silver  are  different  as  to  their  buoyancy 
in  water,  or,  as  we  say  now,  as  to  the  specific  gravity.  So 
taking  a  weight  of  gold  equal  to  the  weight  of  the  crown; 
and  comparing  it  with  the  crown  as  to  buoyancy,  he  could 
prove  whether  the  crown  was  pure  gold  or  not. 
63 


64  ELEMENTARY  SCIENCE 

Whether  Archimedes  discovered  the  goldsmith  to  be 
a  fraud  or  not  is  not  important.  He  had  made  a  discovery 
far  more  important  than  that.  He  had  discovered  what 
is  known  even  to-day  as  Archimedes'  principle:  the  prin- 
ciple that  a  body  immersed  in  a  liquid  is  buoyed  up  by  a  force 
equal  to  the  weight  of  the  liquid  that  it  displaces. 

If  the  body  weighs  more  than  the  amount  of  liquid  dis- 
placed, it  will  sink;  if  less,  it  will  float.  A  floating  body 
sinks  to  such  depth  in  the  liquid  that  the  weight  of  the 
liquid  displaced  equals  the  weight  of  the  body.  This  ap- 
plication of  Archimedes'  principle  you  can  easily  prove. 
Take  a  piece  of  wood  coated  with  paraffin  to  prevent  its 
absorption  of  water.  Fill  a  pitcher  with  water  until  it 
overflows  at  the  spout.  Then  float  the  wood  in  this  water 
and  catch  all  that  runs  out  of  the  spout.  The  water  thus 
obtained  and  the  piece  of  wood  should  weigh  the  same. 
Try  the  same  thing  with  a  piece  of  ice,  making  allowance 
for  the  melting  of  the  ice.  Is  it  true  that  seven  times  as 
much  of  an  iceberg  is  under  water  as  above  it  ? 

By  means  of  Archimedes'  principle  we  may  also  com- 
pute the  volume  of  solid  objects  of  any  shape.  Weigh 
the  object  in  air,  and  then  weigh  it  under  water.  The 
difference  in  the  two  weights  represents  the  weight  of  the 
water  displaced.  Suppose  this  difference  be  ten  grams. 
We  know  that  ten  grams  of  water  by  weight  is  ten  cubic 
centimeters  of  water  by  volume.  We  also  know  that  the 
volume  of  water  displaced  is  identical  with  the  volume  of 
the  object  submerged.  Therefore,  the  volume  of  the  object 
is  ten  cubic  centimeters.  In  other  words,  the  difference  in 
grams  between  the  weight  of  an  object  in  air  and  its  weight 
in  water  is  equal  to  the  wlume  of  that  object  in  cubic  centi- 
meters. 


ARCHIMEDES'  PRINCIPLE  65 

Water-Pressure.  —  The  pressure  of  a  liquid  is  exerted 
equally  in  every  direction  against  the  sides  and  bottom  of 
whatever  contains  it.  Suppose  there  is  a  tank  of  water  at 
the  top  of  a  house,  with  a  pipe  leading  down  from  it  and 
branching  off  to  different  rooms.  Now  if  you  turn  on  the 
water  at  a  tap  directly  below  the  tank,  will  it  run  out 
with  any  greater  force  than  from  a  tap  many  feet  away  to 
one  side?  You  know  that  it  will  not.  This  illustrates 
the  principle  stated  above. 

It  is  possible  to  take  advantage  of  this  principle  in  per- 
forming various  kinds  of  work.  Since  liquids  thus  trans- 
mit pressure  equally  in  every  di- 
rection against  the  walls  of  their 
containers,  it  is  possible  to  use 
water  as  a  means  for  multiply- 
ing the  area  of  pressure.  Thus  in 
the  hydraulic  press  (see  Fig.  28) 
a  pressure  applied  at  ab  will  be 

equally  exerted  by  the  surface  at  AB,  as  well  as  by  all  the 
other  contained  surfaces.  If  there  is  a  pressure  of  one 
pound  at  ab,  there  will  be  a  pressure  at  AB  of  as  many 
pounds  as  the  area  of  ab  is  contained  in  the  area  of  AB. 
This  is  what  is  meant  by  multiplying  the  pressure.  If  AB 
is  ten  times  as  large  as  ab,  then  one  pound  laid  on  ab  will 
balance  ten  pounds  laid  on  AB. 

Thus  we  have  a  multiplication  of  pressure,  but  this 
must  not  be  confused  with  multiplication  of  work.  If 
the  piston  ab  is  pushed  down  one  inch,  will  the  piston  AB 
rise  one  inch  ?  Certainly  not.  For  one  thing,  there  would 
not  be  enough  water  to  fill  such  an  addition  to  the  space. 
The  piston  AB  will  rise  only  in  proportion  to  the  difference 
between  its  area  and  the  area  of  ab.  If  it  be  ten  times  as 


66 


ELEMENTARY  SCIENCE 


large  as  ab,  and  ab  be  forced  down  one  inch,  then  AB  will 
be  forced  up  one- tenth  of  an  inch.  The  pressure  at  A  B  is 
ten  times  as  great  as  the  pressure  at  ab,  but  it  works  through 
only  one-tenth  of  the  distance,  thus  fulfilling  the  law  of 
conservation  of  energy.  You  have  heard  of  the  law  of  the 
indestructibility  of  matter.  You  know 
that  matter  may  be  changed,  but  it 
cannot  be  produced  or  destroyed.  So 
energy  or  work  can  be  changed;  the  area 
over  which  it  operates  can  be  multiplied. 
But  the  work  done  cannot  be  multiplied. 
Energy  like  matter  can  be  changed  in 
form,  as  from  electricity  to  light,  but, 
like  matter,  it  can  neither  be  produced 
nor  destroyed.  However  wonderful  a 
machine  may  be,  the  amount  of  work  it  does 
is  never  greater  than  the  amount  of  energy  it 
takes  to  make  it  work.  The  work  taken  out 
cannot  be  greater  than  the  work  put  in. 
Water-pressure  is  used  in  the  operation 
of  elevators  (see  Fig.  29).  Water  forced 
bto  the  cylinder  C  drives  back  the  piston 
and  produces  a  pull  on  the  cable.  The 
-elevator  goes  up.  The  operator  stops  the  car  by  closing 
the  valve  that  shuts  off  the  water  from  the  water-main. 
He  lowers  it  by  opening  a  valve  that  permits  the  water 
in  the  cylinder  to  run  off  into  the  sewer.  City  water-power 
is  also  commonly  used  to  operate  pumps  which  pump 
soft  water  into  tanks  at  the  top  of  houses. 

Water-Works.  —  By  water- works  we  mean  constructions 
ior  the  collection  and  distribution  of  water  for  towns  and 


ARCHIMEDES'  PRINCIPLE  67 

cities.  A  surprising  amount  of  water  is  needed  for  this 
purpose.  The  daily  consumption  of  water  per  person  for 
certain  cities  has  been  estimated  in  gallons  as  follows: 
New  York  79,  Chicago  140,  Philadelphia  132,  St.  Louis  72, 
Boston  80,  Washington  187.  In  European  cities  it  is 
much  less,  because  of  restrictions.  The  question  of  water- 
supply  is  often  one  of  the  most  serious  questions  which  a 
city  has  to  face. 

The  purity  of  water  is  just  as  important  to  a  city  as 
its  amount,  for  many  of  the  commonest  and  most  serious 
diseases,  especially  typhoid  fever,  are  caused  by  impure 
water.  In  many  cities  the  death-rate  has  been  much 
lowered  by  purifying  the  water-supply.  Thus  in  Chicago 
after  the  opening  of  the  drainage  canal,  which  purified 
the  supply  of  water  from  Lake  Michigan,  the  rate  of  death 
from  typhoid  alone  was  reduced  by  two-thirds.  This 
represents  a  saving  of  nearly  a  thousand  lives  a  year. 

Ancient  Rome  had  an  elaborate  system  of  water-works. 
Portions  of  its  aqueducts  still  stand;  they  were  very  excel- 
lent constructions  (aqua,  water;  duco,  lead).  These  aque- 
ducts were  for  the  purpose  of  leading  water  across  valleys 
and  lowlands  from  its  source  in  the  highlands.  Such  a 
system  is  called  a  gravity  system;  it  depends  wholly  upon 
the  force  of  gravity  to  secure  the  desired  flow  of  water. 
Gravity  systems  of  water-works  are  largely  used  to-day. 
However,  it  is  not  necessary  to  build  high  aqueducts  across 
valleys  as  it  was  in  the  days  of  ancient  Rome.  The  water 
may  be  carried  across  valleys  and  under  rivers  in  iron 
pipes,  which  rise  and  re-enter  the  aqueduct  on  the  opposite 
side.  As  long  as  the  rise  is  not  higher  than  the  head  of  the 
water-supply,  the  water  will  continue  to  flow,  however  deep 
its  plunge.  The  ancients  could  not  use  this  plan  on  ac- 


68  ELEMENTARY  SCIENCE 

count  of  their  limited  knowledge  of  iron-working.  The 
High  Bridge  over  the  Harlem  River  in  New  York  City  was 
built  to  serve  as  an  aqueduct  as  well  as  a  bridge. 

The  pipes  which  convey  water  through  city  streets  are 
called  mains.  The  smaller  pipes  which  enter  the  houses 
are  called  service-pipes.  Water-meters  are  installed  in  the 
houses  to  record  the  amount  of  water  used.  The  point 
at  which  a  service-pipe  goes  through  the  foundation  wall 
of  a  house  is  often  the  point  of  greatest  danger  from  freez- 
ing. So  it  is  desirable  to  have  the  service-pipe  enter  the 
house  on  the  sunny  side.  It  is  also  desirable  to  have  the 
pipes  protected  by  wrapping  where  they  are  most  exposed 
to  cold.  The  depth  at  which  service-pipes  must  be  laid 
to  prevent  bursting  from  freezing  depends  on  the  climate 
and  the  nature  of  the  soil.  Loose,  gravelly  soil  freezes 
much  deeper  than  more  compact  soil. 

In  low  and  level  country  stand-pipes  are  commonly  used 
in  order  to  give  gravity  flow.  The  water  is  pumped  up 
into  them.  But  in  cities  with  high  ground  near  them,  the 
water  is  pumped  into  high-lying  reservoirs  instead. 

When  the  water  comes  from  a  river,  or  from  some  other 
source  of  doubtful  purity,  filters  are  used  to  remove  the 
impurities.  A  filter  is  a  construction  of  porous  material 
through  which  the  water  is  strained.  Sand,  gravel,  char- 
coal, and  porcelain  are  substances  used  for  filters.  Even 
filtered  water,  however,  may  contain  disease  germs,  and 
when  there  is  any  doubt  about  its  purity,  it  should  be 
boiled.  This  destroys  the  germs. 


ARCHIMEDES'  PRINCIPLE  69 


QUESTIONS 

1.  What  is  Archimedes'  principle? 

2.  Explain  the  principle  of  the  hydraulic  press. 

3.  If  the  smaller  surface  on  a  hydraulic  press  is  two  inches  square, 

and  the  larger  surface  is  a  foot  and  a  half  square,  how  many 
pounds  pressure  must  be  exerted  on  the  smaller  surface  to 
balance  a  weight  of  fifty-three  pounds  laid  on  the  larger 
surface?  If  the  piston  at  the  smaller  surface  were  pushed 
down  twelve  and  one-quarter  inches,  how  much  would  the 
weight  on  the  larger  surface  rise? 

4.  What  is  meant  by  "conservation  of  energy"? 

5.  Explain  how  a  hydraulic  elevator  works. 

6.  How  would  you  construct  a  city  water-system? 

7.  What  is  a  filter? 


CHAPTER  DC 
WATER-POWER  AND  STEAM-POWER 

The  power  of  falling  water  has  long  been  used  by  man 
to  do  various  kinds  of  work.  The  grinding  of  grain  was 
one  of  the  most  tiresome  of  early  occupations.  No  wonder 
that  some  tired  grinder  put  his  wits  to  work  to  find  a  way 
of  getting  out  of  this  endless  job.  Grain  was  ground  in 
ancient  times  between  stones,  as  it  is  even  to-day  in  some 
primitive  countries.  So  the  grinding  together  of  stones 
by  a  waterfall  may  have  been  the  thing  that  suggested 
having  the  water  do  this  work.  Water-wheels  were  de- 
vised, and  set  up  in  places  where  falling  water  could  be 
easily  diverted  to  run  over  them.  Dams  were  built  to 
impound  the  water  and  make  its  flow  over  the  water-wheel 
constant.  The  revolving  shaft  of  the  water-wheel  turned 
the  grinding  stones  of  the  early  mills  (see  Fig.  30). 

Many  American  cities  are  where  they  are  because  they 
grew  up  around  old  mills.  Along  the  Connecticut  River 
in  New  England  there  are  many  manufacturing  cities. 
They  are  there  simply  because  the  many  falls  and  rapids 
of  the  river  furnish  the  power  which  runs  their  mills. 
To-day,  however,  men  have  learned  to  transform  water- 
power  into  electric  power,  and  thus  to  transmit  it  over  wires 
to  cities  many  miles  away.  So  mills  no  longer  need  to 
be  near  the  falling  water  which  furnishes  power  to  operate 
their  industries.  Niagara  Falls  is  the  source  of  the  power 
that  runs  the  street-cars  of  cities  hundreds  of  miles  away. 
70 


WATER-POWER  AND  STEAM-POWER 


The  Mississippi  River,  by  means  of  the  great  dam  at  Keo- 
kuk,  Iowa,  is  made  a  source  of  power  for  St.  Louis  and 
many  other  cities.  Do  you  know  any  other  examples 
of  falling  water  that  has  been  "harnessed"  by  man  so  that 
it  is  the  source  of  energy  that  does  work  many  miles  away  ? 
When  you  were  studying  the  movements  of  water, 
and  its  many  changes  from  one  form  into  another,  you 
learned  what  is  meant  by  the  indestructibility  of  matter. 
Now,  in  thinking  about  the 
various  ways  in  which  water- 
power  may  be  used,  you  can 
see  that  a  similar  principle 
is  at  work.  Energy,  which 
means  the  power  to  do  work, 
is  a  thing  that  can  be  changed, 
but  it  cannot  be  destroyed. 
Matter  and  energy  are  the 
two  great  things  that  make  up 
the  world  and  neither  of  them 
can  be  destroyed.  They  keep 
changing  all  the  time,  but 
they  are  not  used  up.  Sup- 
pose you  carry  a  trunk  up- 
stairs. You  may  think  you  have  used  up  a  good  deal  of 
energy  in  doing  this.  But  the  energy  that  you  used  in 
carrying  that  trunk  up-stairs  still  exists,  as  you  would  see 
if  you  let  the  trunk  slip  and  it  fell  down-stairs.  In  other 
words,  energy,  like  matter,  may  be  easily  transformed  (as 
water-power  into  electricity),  but  it  cannot  be  destroyed. 
You  may  not  be  able  to  understand  this  completely  now, 
but  you  will  see  more  and  more  illustrations  of  it  as  you  go 
on  with  your  work. 


FIG.  30. — Diagram  illustrating  how 
the  water-wheel  works. 


72 


ELEMENTARY  SCIENCE 


Water-power  may  be  defined  as  power  derived  from 
falling  water  and  converted  (by  means  of  motors)  into 
useful  work.  A  motor  is  any  apparatus  which  changes 
(transforms)  one  form  of  energy 
into  another.  Steam  and  gaso- 
lene engines,  windmills,  and 
electric  dynamos,  as  well  as 
water-wheels,  are  forms  of  mo- 
tors. 

It  is  quite  simple  to  compute 
the  amount  of  power  which 
may  be  generated  by  water  at 
a  given  point.  The  number  of 
pounds  of  water  that  flows  in  a 
given  time  is  multiplied  by  the 
number  of  feet  of  the  fall.  This 
gives  the  theoretical  amount  of 
power.  From  this  amount,  of 
course,  deductions  must  be 
made,  for  there  is  always  con- 
siderable loss  of  power  in  the 
processes  of  transformation  and 
transmission.  By  transmission 
is  meant  the  sending  of  the 
power.  This  may  be  done  by 
geared  shafts,  or  by  belts,  or, 
in  the  case  of  electricity,  by 
means  of  wires. 

The  power-plant  at  the  famous  Keokuk  dam  illustrates 
quite  simply  the  most  modern  and  scientific  method  of 
getting  power  from  falling  water.  This  is  the  turbine 
method  (turbo,  whirl).  In  a  turbine  motor  the  falling 


FIG.  31.— The  turbine  in  action. 


WATER-POWER  AND   STEAM-POWER 


73 


water  enters  a  cylinder  in  streams  that  strike  the  blades 
of  a  revolving  shaft  hi  such  a  way  that  nearly  all  the 
power  of  the  falling  water  is  utilized  in  spinning  the  shaft 
(see  Figs.  31  and  32).  By  this  means  only  about  twenty- 


FIG.  32. — The  machinery  of  the  turbine. 

five  per  cent  of  the  theoretical  energy  is  lost,  which  is  much 
less  than  the  amount  lost  in  the  use  of  the  older  types  of 
water-wheels.  Turbines  are  also  used  in  generating  power 
by  means  of  steam-pressure.  Many  modern  steamships  use 
turbine  motors. 
It  has  been  estimated  that  the  rivers  of  the  United 


74  ELEMENTARY  SCIENCE 

States  can  furnish  about  two  hundred  million  horse-power, 
while  the  amount  utilized  now  is  less  than  two  million. 
Rapid  development  of  this  water-power  is  being  made. 
Each  year  many  factories,  for  the  sake  of  economy,  close 
down  their  own  steam-power  plants  and  buy  electric 
power,  which  is  delivered  to  them  from  plants  like  the  one 
at  Keokuk. 

A  horse-power  is  the  standard  unit  in  measuring  the  rate 
of  mechanical  work.  It  is  the  amount  of  work  done  in 
raising  thirty-three  thousand  pounds  one  foot  in  one 
minute.  This  unit  was  devised  by  Watt  from  observations 
made  on  the  amount  of  work  done  by  strong  dray-horses. 
He  found  that  the  average  dray-horse  is  able  to  work  at 
the  rate  of  twenty-two  thousand  foot-pounds  per  minute, 
foot-pound  meaning  the  amount  of  work  done  in  raising 
one  pound  one  foot.  He  arbitrarily  increased  this  amount 
by  one-half,  and  used  it  to  measure  the  power  generated 
by  the  steam-engine  he  had  invented.  It  has  been  used 
ever  since. 

Water-Power  and  Gravity.  —  You  have  noted  that  the 
work  that  water  does,  or  can  be  made  to  do,  depends 
upon  the  fact  that  it  falls.  Now  before  anything  falls  it 
must  be  lifted.  Then,  when  it  falls,  it  falls  with  precisely 
the  same  force  as  was  required  to  lift  it.  Here  we  have  a 
statement  of  the  law  of  gravity,  by  which  we  mean  the  ten- 
dency of  bodies  to  fall  toward  the  center  of  the  earth.  (Gravi- 
tation means  the  tendency  of  all  bodies  to  attract  each  other. 
Thus  gravity,  which  is  the  most  familiar  example  of  gravi- 
tation, is  a  case  of  gravitation  in  which  the  earth  is  the 
principal  body  involved.) 

This  law  of  gravity  has  been  of  great  aid  to  man  hi  many 


WATER-POWER  AND   STEAM-POWER  75 

of  his  devices.  It  aided  him  in  his  first  defenses  against 
his  enemies.  He  climbed  trees  to  escape  from  those  of 
his  enemies  that  could  not,  by  climbing,  overcome  the  law 
of  gravity.  Then  he  built  stockades  and  walls  over  which 
his  human  enemies  could  not  climb.  He  carried  large 
stones  to  the  tops  of  walls  where  they  could  be  conve- 
niently dropped  upon  the  heads  of  those  who  came  to  at- 
tack him.  Thus  he  used  the  force  of  gravity  to  crush  his 
enemies. 

But  nowadays  falling  water  is  much  more  important  to 
man  than  falling  stones.  In  ancient  days  slaves  did  the 
work  of  carrying  up  the  stones.  What  does  the  work  of 
carrying  up  the  water?  What  is  the  force  that  takes  the 
water  from  the  seas  and  lakes  and  carries  it  up  to  the 
hills,  whence  it  runs  back  to  the  seas  again,  and,  in  running, 
turns  the  wheels  of  ten  thousand  mills?  Evidently  we 
must  think  again  of  that  great  water  cycle  of  nature  de- 
scribed in  Chapter  III.  And  we  must  ask  ourselves 
what  is  the  force  behind  evaporation  ?  We  noted  in 
Chapter  I  that  increase  of  heat  hastens  evaporation. 
Now  can  it  be  that  it  is  the  heat  of  the  sun  that 
does  this  great  work  of  lifting  water  up?  Is  it  the  sun 
that  we  have  to  thank  for  getting  the  water  in  such  po- 
sition that  its  fall  does  work  for  us  ?  Does  the  sun  then 
serve  us  somewhat  as  the  slaves  of  old  served  their  masters 
when  they  carried  stones  up  to  high  places  ?  We  shall  see. 

Steam-power  is  like  water-power  in  that  water  (in  the 
form  of  gas)  is  used.  It  is  used  to  push  against  something. 
In  the  case  of  steam-engines  this  pushing  or  pressure  is 
the  pressure  of  steam  in  a  cylinder  against  a  piston  (see  Fig. 
i).  The  compressed  steam  forces  the  piston  through  the 
cylinder,  then  this  steam  is  released.  Other  steam  imme- 


76 


ELEMENTARY  SCIENCE 


diately  admitted  on  the  other  side,  by  means  of  valves, 
forces  the  piston  back.  Thus  a  powerful  motion  is  pro- 
duced. Thus  locomotives  are  driven  at  high  rates  of  speed 
and  draw  heavy  trains.  You  have  seen  the  rapid  motion  of 
the  shaft  of  locomotives  which  runs  from  the  cylinder  to  the 
great  driving-wheels.  The  force  which  turns  the  wheels 
comes  from  that  cylinder  in  which  the  piston  moves  rapidly 
to  and  fro,  the  pressure  of  steam  against  it  being  controlled 


FIG.  33. — The  locomotive. 

by  the  engineer.  You  have  seen  the  used  steam  as  it 
escapes,  hissing,  from  the  ends  of  the  cylinders.  Some  of 
it  goes  up  the  stack  and  escapes  with  the  smoke.  Study 
Fig.  33- 

A  steam-engine  may  be  defined  as  a  machine  for  convert- 
ing heat  energy  into  mechanical  motion  through  the  medium 
of  steam.  It  consists  of  the  fire-box  and  boiler  (where  the 
steam  is  generated)  and  the  engine  proper,  where  the 
steam  acts  on  a  piston,  producing  motion.  Machines  in 
which  the  steam  acts  on  blades  set  on  a  rotating  wheel 
instead  of  on  pistons  are  called  steam-turbines,  which  you 
have  already  noted, 


WATER-POWER  AND  STEAM-POWER  77 

Water-Power  and  Steam-Power  Compared.  —  They  are 
alike  in  that  they  both  use  water.  But  let  us  note  an 
apparent  difference  as  to  the  force  behind.  You  have  noted 
that  the  force  behind  water-power  is  the  force  of  gravity. 
There  is  no  water-power  unless  the  water  falls.  Thus 
it  is  gravity,  acting  on  water,  that  is  the  real  force  that 
turns  the  turbines,  that  spins  the  generators,  that  gener- 
ates the  electricity,  that  lights  the  electric  lamp  by  whose 
light,  even  now,  you  may  be  reading  these  words. 

In  steam-power,  however,  it  is  heat  that  is  the  force 
behind.  It  is  heat  that  expands  the  water  into  steam, 
and  then  expresses  itself  in  pressure  if  this  steam  is  con- 
fined. This  heat,  in  turn,  is  generated  by  the  burning  of 
fuel.  Whether  the  fuel  be  coal  or  wood,  its  source  is 
plants,  for  coal  is  a  formation  of  ancient  plants  (see  page 
49).  Now  since  we  can  release  it  by  burning,  there  must 
be  energy  in  the  wood  and  coal.  Where  did  they  get  it? 
They  got  it  from  the  sunlight  which  once  shone  on  the 
leaves  of  the  plants  of  which  they  were  a  part.  So,  if  it 
be  that  the  wire  from  your  electric  lamp  runs  to  a  steam- 
operated  electric  plant,  it  is  the  force  of  the  sunlight  of 
ancient  days  which  is  now  expressing  itself  in  the  light 
you  read  by.  Think,  then,  how  these  two  ancient  forces, 
gravity  and  sunlight,  serve  us  to-day.  Whether  it  be  the 
light  by  which  we  read,  the  trains  by  which  we  travel,  the 
wires  over  which  we  send  our  messages,  the  automobiles 
in  which  we  ride,  or  any  one  of  a  thousand  other  of  our 
modern  conveniences,  we  can  trace  all  back  to  the  source 
of  their  power,  and  find  it  in  one  or  the  other  of  these  great 
servants  of  mankind,  the  pull  of  this  old  earth  we  live  on 
and  the  sunlight  which  shines  upon  it.  Yet  we  should 
remember  that  before  the  "pull  of  earth"  can  act  on  water, 


78  ELEMENTARY  SCIENCE 

and  thus  give  us  what  we  call  water-power,  the  water 
must  be  lifted  to  the  high  parts  of  the  land  from  which  it 
flows  down.  The  force  which  does  this  is,  as  has  been 
suggested,  the  heat  of  the  sun.  So  it  is  to  the  sun,  to  its 
heat  in  one  case  and  to  its  light  in  the  other,  that  we  must 
finally  go  back  when  we  are  seeking  an  explanation  of 
water-power  and  of  steam-power.  Thus  we  see  that  here, 
as  everywhere  in  nature,  we  have  an  endless  chain  of  causes 
and  effects,  and  that  the  sun  is  the  great  original  source  of 
the  energy  we  see  displayed  on  earth. 

QUESTIONS 

1.  Explain  why  waterfalls  are  useful. 

2.  Where  did  the  energy  that  runs  your  watch  come  from? 

3.  How  would  you  compute  the  power  that  could  be  generated 

by  Niagara  Falls? 

4.  How  is  power  transmitted? 

3.  Explain  the  turbine.    Why  is  it  better  than  the  old-fashioned 
water-wheel  ? 

6.  What  is  a  horse-power?    A  foot-pound? 

7.  What  originally  furnished  the  energy  of  the  waterfall  ? 

8.  Explain  the  steam-engine. 

9.  What  is  the  difference  between  a  water-turbine  and  a  steam- 

turbine  ? 

10.  What  put  the  energy  into  coal? 

11.  Where  did  all  energy  come  from  originally? 


CHAPTER  X 
WATER  AND  AGRICULTURE 

The  Importance  of  Water  to  Plants. —  You  know 
how  important  water  is  to  plants  and  also  that  water  has 
great  effect  in  determining  where  men  live.  This  is  due  to 
the  need  of  it  for  agriculture  (food-production)  quite  as 
much  as  to  the  need  of  it  for  direct  use  by  man,  and  in  con- 
nection with  transportation.  Good  crops  depend  upon 
water,  soil,  and  climate.  Soil  and  climate  suitable  for  agri- 
culture are  much  more  wide-spread  than  suitable  water- 
supply.  Hence  water-supply  is  the  chief  factor  in  modern 
times  in  determining  the  location  of  farms.  Until  quite 
recently  the  availability  of  a  market  for  the  crops  was  quite 
as  important  a  factor,  but  the  modern  development  of  trans- 
portation has  very  largely  solved  this  problem. 

To  insure  good  crops  of  hay  and  grain,  a  water-supply 
equivalent  to  at  least  ten  inches  of  rainfall  a  year  is  needed. 
Also  this  supply  must  be  distributed  throughout  the 
growing  season  of  the  plants;  it  will  not  do  if  it  all  comes 
at  once  in  heavy  rains  followed  by  long  periods  of  drought. 

It  is  easy  to  demonstrate  that  water  evaporates  from 
the  leaves  of  plants.  Place  a  bell-jar  over  a  potted  gera- 
nium, covering  pot  and  soil  with  sheet  rubber;  or  a  tum- 
bler over  a  single  leaf  whose  end  is  in  covered  water. 
Droplets  or  a  film  of  moisture  will  soon  collect  on  the  glass. 
This  is  evidently  a  result  of  plant-evaporation,  a  process 
which  is  technically  called  transpiration. 
79 


8o  ELEMENTARY  SCIENCE 

The  amount  of  water  transpired  by  growing  crops  has 
frequently  been  estimated.  It  has  been  found  that  in 
the  production  of  one  ton  of  well-dried  hay,  over  three 
hundred  tons  of  water  are  used.  Corn  which  produces 
fifty  bushels  to  the  acre  uses,  during  its  life,  an  acre  of 
water  ten  inches  deep. 

Irrigation.  —  Agriculture  does  not  depend  for  water  on 
rain  alone.  Doubtless  you  have  heard  of  irrigation.  It 
is  the  process  of  watering  land  by  artificial  means.  It 
is  an  ancient  art.  It  was  practised  in  the  Nile  Valley  in 
the  days  of  the  Pharaohs,  even  before  the  pyramids  were 
built,  and  even  to-day  we  find  highly  developed  irrigation 
systems  built  and  operated  by  savage  tribes.  They  are 
found  in  mountainous  regions  where  agriculture  requires 
the  building  of  terraces  as  well  as  the  conveyance  of  water 
by  ditches. 

The  modern  development  of  irrigation  in  the  United 
States  is  very  extensive.  We  have  in  the  West  vast  areas 
of  desert  land;  the  climate  is  excellent  for  agriculture  and 
the  soil  is  usually  fertile.  All  that  is  lacking  to  make 
these  lands  "blossom  as  the  rose"  is  water;  water  supplied 
throughout  the  growing  season.  To  thousands  of  such 
acres  in  some  Western  States,  water  has  been  supplied. 
It  has  been  supplied  by  ditches  from  rivers  and  from 
mountain  reservoirs,  and  now  some  of  this  irrigated  land 
is  the  most  fertile  and  valuable  land  that  we  have;  often 
when  developed  it  sells  for  more  than  a  thousand  dollars 
an  acre.  In  Arizona  and  California  you  may  see  orange 
groves,  or  rich  and  beautiful  fields  of  alfalfa,  while  just  over 
the  fence  from  them  is  the  desert,  whose  only  plants  are 
sagebrush  and  cactus  and  the  like.  The  soil  is  the  same. 


WATER  AND  AGRICULTURE 


8l 


The  sunlight  is  the  same.  The  heat  is  the  same.  The 
only  difference  is  the  "magic  touch"  of  water  (Figs.  34 
and  35). 

Irrigation  projects  frequently  involve  the  expenditure 
of  great  sums  of  money  on  the  necessary  dams  and  ditches. 
It  may  be  necessary  to  bore  tunnels  to  carry  the  water 


FIG.  34.— Some  of  our  Western  land  which  has  not  been  irrigated.— After  U.  S. 
Reclamation  Bureau. 

from  the  mountain  reservoir  to  the  thousands  of  acres 
which  it  may  reclaim  from  the  desert.  This  great  expendi- 
ture of  money,  combined  with  the  fact  that  the  land  to 
be  reclaimed  is  in  most  cases  government  land,  makes  it 
natural  that  we  have  in  the  United  States  a  government 
Reclamation  Service.  This  important  branch  of  the 
government  makes  the  careful  preliminary  surveys  which 
are  necessary,  and  then  proceeds,  as  funds  are  made 
available  by  Congress,  to  build  irrigation  works  at  what 


82 


ELEMENTARY  SCIENCE 


seem  the  most  desirable  places.     The  land  which  is  made 
fertile  by  such  works  is  then  sold  to  settlers. 

Between  1902  and  1915  more  than  three  million  acres 
were  reclaimed  by  the  government  by  irrigation.  Plans 
have  been  completed  and  part  of  the  work  done  to  reclaim 
thirty  million  more.  In  1914  homesteaders  of  seven  hun- 


FIG.  35.— The  same  sort  of  land  as  in  Fig.  34  after  irrigation.— After  U.  S.  Rec- 
lamation Bureau. 

dred  thousand  acres  of  this  land  got  average  crop  returns 
of  twenty-five  dollars  per  acre.  The  average  cost  of  the 
perpetual  water-right  is  forty  dollars  per  acre,  the  home- 
steader being  given  twenty  years  in  which  to  pay  this  in 
instalments  without  interest.  Complete  information  con- 
cerning the  Reclamation  Service  may  be  obtained  by  writ- 
ing to  the  United  States  Geological  Survey  at  Washington. 
Why  is  it  called  the  Reclamation  Service  and  not  the 
Irrigation  Service?  This  is  because  it  does  not  reclaim 


WATER  AND  AGRICULTURE  83 

land  by  irrigation  alone.  There  is  much  land  in  the 
United  States  which  is  rendered  unfit  for  agriculture  not 
by  too  little  water,  but  by  too  much  water.  Such  land  is 
reclaimed  by  drainage.  We  have  more  than  one  hundred 
million  acres  of  swamp-land,  most  of  which  by  drainage 
can  be  made  fit  for  farms.  The  soil  is  very  fertile,  once  the 
excess  of  water  is  removed.  In  Florida  thousands  of  acres 
have  been  reclaimed  by  drainage,  and  smaller  drainage 
projects  are  found  practically  all  over  the  United  States. 

Drainage.  —  We  note  then  that  though  water  is  neces- 
sary to  plants,  there  may  be  "too  much  of  a  good  thing." 
So  scientific  farming  involves,  among  many  other  things, 
the  effort  to  secure  for  crop  plants  the  optimum  (best)  wa- 
ter-supply. It  is  for  this  reason  that  good  farmers  lay  tile- 
drains  in  their  fields  where  there  is  need  for  them.  Let 
us  note,  however,  that  such  drains  benefit  the  plants  in 
other  ways  than  merely  by  carrying  off  the  excess  of  water. 
In  the  excess  of  water  they  carry  off  excretions  from  the 
roots  of  previous  crops  which,  if  not  removed,  make  the 
soil  less  fertile.  They  also  indirectly  permit  more  air 
to  get  into  the  soil,  which  is  important  to  plants.  The 
increase  in  the  porosity  of  the  soil  caused  by  draining  off 
the  excess  of  water  also  permits  heat  to  penetrate  it  more 
rapidly;  the  wanning  up  of  the  soil  in  spring  is  especially 
important  in  connection  with  the  germination  of  seeds. 
These  incidental  benefits  of  drainage  are  so  important  that 
it  has  been  found  desirable  to  provide  artificial  drainage 
even  for  irrigated  farms. 

Water  in  the  Soil.  —  The  amount  of  water  which  is  held 
by  soil  depends  on  what  kind  of  soil  it  is.  More  specifically, 


84  ELEMENTARY  SCIENCE 

it  depends  on  the  size  of  the  soil  grains.  Clay  has  greater 
retentive  capacity  for  water  than  sand  because  its  grains 
are  much  smaller.  Liquids  have  a  tendency  to  adhere 
to  the  surface  of  substances  with  which  they  are  in  con- 
tact, even  in  opposition  to  the  force  of  gravity.  So  each 
soil  grain  holds  to  its  surface  a  film  of  water;  this  is  known 
as  the  water  of  adhesion.  Therefore,  the  greater  the  num- 
ber of  soil  grains  in  a  given  volume  of  soil,  the  greater  the 
total  area,  and  the  greater  the  amount  of  water  of  adhesion. 
The  total  surface  of  a  cubic  foot  of  the  tiny  grains  of  clay 
is  very  much  greater  than  the  total  surface  of  a  cubic  foot 
of  the  much  larger  grains  of  sand. 

In  your  study  of  wells  (see  page  46),  you  learned  that  a 
layer  of  clay  is  often  very  important  as  a  barrier  which 
prevents  the  escape  of  water  through  it.  Now  let  us  note 
that  if  this  clay  is  moist,  which  it  inevitably  is,  if  in  con- 
tact with  another  water-containing  stratum,  then  it  be- 
comes much  more  impenetrable  to  water  than  if  it  were 
dry.  Due  to  the  minuteness  of  its  grains,  and  its  conse- 
quent high  powers  of  surface  tension,  it  holds  the  water 
that  gets  into  it  with  greater  tenacity,  and  nothing  is  more 
impervious  (impassable)  to  water  than  a  layer  which  is 
already  filled  with  water. 

Roots  and  Water.  —  You  have  just  noted  that  the  fine- 
ness of  soil  grains  multiplies  the  water-retaining  capacity 
of  soil.  Similarly,  the  increase  of  water-absorbing  surfaces 
multiplies  the  water-absorbing  capacity  of  roots.  This 
increase  of  water-absorbing  surface  roots  secure  by  means 
of  root-hairs.  On  the  roots  of  seedlings,  such  as  those  of 
corn  or  radish,  these  hairs  are  so  dense  that  they  form  a 
fuzz,  as  you  may  have  seen  (see  Fig.  36).  When  you 


WATER  AND  AGRICULTURE 


pull  up  a  plant  by  the  roots,  you  do  not  find  these  hairs. 
They  are  so  delicate  and  so  closely  adherent  to  the  soil 
grains  that  they  break  off  and  remain  in  the  soil.  You 
can  see  that  the  absorptive  surface 
of  roots  is  multiplied  very  many 
times  by  these  millions  of  hairs. 

You  should  also  note  that  roots,  in 
their  growth,  are  positively  attracted 
by  water;  they  grow  toward  it,  and 
when  the  water-supply  is  deeper  than 
usual  plants  may  develop  an  unusual 
depth  of  roots.  In  fact,  the  growth 
of  roots  seems  to  be  chiefly  a  sort  of 
groping  and  searching  in  the  soil  for 
water.  If  you  sprout  seeds  in  a 
suspended  wire  cage  containing  damp 
moss,  the  rootlets  that  grow  down 
through  the  bottom  of  the  cage  will  turn  and  grow  up 
again  toward  the  water  above  them. 

QUESTIONS 

1.  How  can  you  prove  that  water  evaporates  from  leaves? 

2.  What  is  irrigation? 

3.  What  kinds  of  work  are  done  by  the  United  States  Reclamation 

Service  ? 

4.  Why  can't  you  grow  a  plant  in  a  bucket  of  water? 

5.  What  kind  of  soil  holds  the  most  water?     Why? 

6.  What  are  root-hairs,  and  of  what  advantage  are  they  to  the 

plant  ? 


.  36. — -Young  root-tips 
ith  root  hairs;  A  shows 
how  the  soil  particles  be- 
come attached. — After 
J.  M.  COULTER. 


CHAPTER  XI 
ORIGIN  OF  SOIL 

Importance  of  Soil.  —  As  life  depends  on  food,  so  food 
depends  on  soil.  We  know  how  necessary  food  is  to  us. 
It  takes  but  little  thought  to  show  that  soil  is  likewise 
necessary  to  food.  Fish  and  other  sea-foods  we  might 
have,  if  there  were  no  soil,  but  what  would  life  be  worth 
if  our  only  food  was  food  from  the  water?  It  is  the  soil, 
with  the  sunlight  and  heat  on  it  and  water  in  it,  that  makes 
life  possible  for  our  crop  plants,  and  it  is  these  crop  plants 
that  make  life  possible  for  us. 

The  soil  covers  the  rocks  and  holds,  firmly  and  nourish- 
ingly  within  itself,  the  roots  of  plants.  It  is  of  the  soil 
we  think  when  we  say  "Mother  Earth,"  for  it  is  the  soil 
which  permits  Earth  to  be  "Mother"  of  us  all,  plants  and 
animals  alike.  It  is  in  the  soil  that  man  worked  in  the 
long  ages  in  which  his  civilization  was  slowly  developing. 
It  is  from  the  soil  he  drew  his  strength;  from  it  he  gained 
the  foundation  of  his  wisdom.  To-day  more  than  one- 
third  of  the  human  race  are  workers  of  the  soil,  and  upon 
their  work  all  the  rest  of  us  depend.  It  is  not  surprising 
that  those  of  us  who  live  in  cities  are  glad  sometimes  to 
"get  back  to  the  soil,"  to  go  to  the  country  and  see  things 
growing  in  the  fields,  to  make  a  garden  of  our  own,  when 
spring  comes,  or  to  plan  to  buy  a  farm.  All  this  seems 
but  a  response  of  our  nature  to  a  sort  of  instinct;  a  sort 
of  instinct  implanted  by  that  long  intimacy  with  the  soil 
that  our  forefathers  had.  So,  however  far  from  farms 

86 


ORIGIN  OF  SOIL  87 

our  work  may  take  us,  we  still  feel  more  or  less  the  instinct 
to  go  and  work  in  the  earth  for  our  living,  and  find  pleasure 
in  the  following  of  that  instinct. 

Formation  of  Soil.  —  You  have  learned  that  all  sub- 
stances are  either  organic  or  inorganic;  and  the  sciences 
are  often  divided  into  those  which  treat  of  organic  things 


FIG.  37. — A  section  of  the  soil,  showing  its  relation  to  the  underlying  rock. 

and  those  which  treat  of  inorganic  things.  But  soil, 
more  than  anything  else  in  the  world,  is  a  thing  in  which 
the  organic  and  inorganic  meet  and  merge.  It  is  composed 
of  mineral  (inorganic)  matter  derived  from  the  rocks 
beneath,  and  of  organic  matter  derived  from  the  organ- 
isms which  live  on  and  in  it.  So  whether  you  are  study- 
ing geology,  an  inorganic  science,  or  botany,  an  organic 
science,  in  both  you  must  study  the  soil. 

That  soil  is  a  mixture,  a  mixture  in  which  organic  and 
inorganic  materials  "meet  and  merge,"  is  quite  evident 
when  we  consider  its  origin,  or  look  at  it  from  top  to  bot- 
tom (see  Fig.  37).  Where  streams  are  eroding  steep 


88  ELEMENTARY  SCIENCE 

banks,  or  where  there  is  a  freshly  made  railroad  cutting, 
you  may  see,  as  in  the  picture,  how  the  underlying  rock 
and  subsoil  gradually  merge  into  the  soil  which  covers 
them.  In  such  places  you  may  readily  see  how  soil  is 
made  in  part  by  the  decay  of  plants  above  and  in  part  by 
the  decay  of  rock  beneath.  The  death  of  things  helps 
produce  it,  while  it,  in  turn,  helps  to  produce  new  living 
things  again;  it  nourishes  the  plants;  they  draw  from  it 
materials  which  they  use  in  growth,  and  which  they  trans- 
form from  what  was  lifeless  and  inorganic  into  what  is 
living  and  organic.  You  have  learned  in  geography  that 
the  wealth  of  countries  depends  a  good  deal  upon  their 
"mineral  resources,"  an  expression  which  makes  you  think 
of  gold  and  silver  and  iron  and  other  valuable  metals. 
But  the  most  valuable  mineral  resource  of  all  is  the  soil. 

To  understand  how  soil  has  been  formed,  we  must  think 
of  the  earth  as  it  was  before  there  was  any  soil.  It  is 
believed  that  at  one  time  water  and  rock  formed  the  entire 
surface  of  the  earth.  There  was  no  soil  and  there  was  no 
vegetation.  But  this  condition  could  not  continue  long. 
There  are  forces  constantly  at  work  in  nature  which  cause 
even  the  hardest  rock  to  crumble  at  the  surface,  and  thus 
the  formation  of  soil  begins.  The  operation  of  these 
forces  on  rock  is  called  rock  weathering,  and  soil  has  been 
called  "rock-meal"  or  "rock-flour"  because  it  is  chiefly 
composed  of  substances  which  were  once  hard  rock.  The 
organic  material  in  soil,  which  comes  chiefly  from  the  decay 
of  fallen  leaves  and  twigs,  is  called  humus  or  mould,  it  is 
dark-colored.  Rich  surface  soil,  especially  in  forests,  is 
largely  composed  of  humus. 

You  have  learned  that  rock  means  any  hard,  inorganic 
part  of  the  earth.  So  the  loose  rock  (gravel,  sand,  etc/ 


; 


ORIGIN  OF  SOIL  89 

which  covers  the  surface  is  called  mantle  rock.    It  forms 
a  sort  of  mantle  over  the  solid  rock  beneath. 

The  weathering  of  rock,  which  leads  to  the  formation 
of  soil,  is  caused  by  various  things,  and  it  proceeds  at 
various  rates  in  different  places.  Some  of  the  principal 
factors  in  rock  weathering  are  the  following: 

I.  Water.  —  You  have  already  noted  what  a  large  part 
running  water  plays  in  the  distribution  of  soil.     It  also 
reduces  pieces  of  rock  to  sand  and  clay  by  grinding  them 
together.    This  is  quite  evident  to  any  one  who  has  watched 
the  effects  of  water  when  streams  are  running  swiftly  or 
waves  are  pounding  on  the  beaches.    But  there  is  quite 
another  and  an  earlier  part  which  water  plays  in   the 
origin  of  soil,  a  part  in  which  movement  of  the  water  is  not 
necessary.     You  know  that  water  expands  when  it  freezes. 
It  expands  about  one- tenth  of  its  volume.    In  this  ex- 
pansion it  exerts  great  force.    You  have  seen  how  this 
force   may   cause    the   bursting   of  pipes.     Similarly,   it 
may  cause  the  bursting  of  rocks.     Crevices  in  rock  may  be 
filled  with  water.    A  freeze  comes  and  the  water  changes 
to  ice.    The  expansion  that  results  may  have  force  enough 
to  enlarge   the  crevices,  splitting  the   rock  still  farther. 
After  a  thaw,  more  water  runs  in.    Then  the  process  may 
be  repeated.     So,  with  alternate  freezings  and  thawings, 
great  masses  of  rock  may  be  broken  up,  and  this  process 
will  be  effective  wherever  water  can  get  into  crevices. 
Evidently  it  will  be  most  effective  where  changes  from  water 
to  ice,  and  the  reverse,  are  most  frequent.     So  in  some  re- 
gions this  process  produces  much  larger  effects  than  in 
others. 

II.  Expansion  and  Contraction.  —  You  know   that  in 
general  heat  causes  substances  to  expand  while  cold  causes 


9o 


ELEMENTARY   SCIENCE 


them  to  contract,  the  expansion  of  water  into  ice  being  a 
conspicuous  exception  to  a  general  rule.  Now  where  rocks 
are  exposed,  as  on  the  sides  of  cliffs  and  in  deserts,  and 
especially  where  there  are  considerable  changes  in  tempera- 
ture between  day  and  night,  the  alternating  contraction 
and  expansion  causes  the  outer  layers  of  rocks  to  break 
and  scale  off  (Fig.  38).  The  surface  of  the  rock,  when  the 
sun  is  shining  on  it,  becomes  hotter  and  more  expanded 


FIG.  38. — Photograph  showing  how  rocks  may  crack  and 
scale  off,  due  to  changes  in  temperature. 

than  the  interior.  This  causes  a  strain  in  the  rock,  and 
starts  the  process  of  breaking.  Then,  at  night,  the  surface 
may  become  colder  and  more  contracted  than  the  part 
just  below  it.  This  causes  more  strain  and  more  break- 
ing, so  that  as  a  result  it  is  common  to  find  that  cliffs 
have  much  loose  rock  on  their  surfaces,  and  the  fallen 
pieces  accumulate  in  great  heaps  at  the  bottom.  Such 
accumulations  of  broken  rock  at  the  foot  of  cliffs  are 
called  talus  (see  Figs.  39,  40).  The  talus  at  the  bases 
of  large  mountains  is  sometimes  hundreds  of  feet  deep. 
This  shattering  of  rock  by  alternate  heating  and  cooling 


ORIGIN  OF  SOIL  91 

is  more  conspicuous  at  high  altitudes  than  at  low  ones. 
Why? 

III.  Chemical  Action.  —  The  changes  in  rock  just  de- 
scribed are  physical  changes.  There  are  also  certain  chem- 
ical changes  constantly  occurring  which  also  contribute 
to  the  reduction  of  rock  to  soil.  Chief  of  these  is  the  proc- 


FIG.  39. — Talus  slope. 

ess  of  oxidation,  which  is  the  chemical  union  of  oxygen 
with  any  other  substances.  Similarly,  hydration  is  the 
chemical  union  of  water  with  any  other  substance.  The 
rusting  of  iron  includes  both  oxidation  and  hydration. 
Now  many  rocks  contain  iron.  When  this  iron  is  exposed 
to  the  atmosphere,  especially  to  a  damp  atmosphere,  it 
becomes  changed  to  iron-rust,  and  this  tends  to  make  the 
rock  crumble. 
You  have  learned  (seepage  52)  how  water  may  "  act  on" 


92  ELEMENTARY   SCIENCE 

limestone  and  sandstone.  This  action  is  a  combination 
of  solution  and  hydration.  You  have  noted  its  effects. 
Similar  effects  are  produced  by  the  oxidation  and  hydra- 
tion of  iron  in  rock;  the  rock  becomes  weakened  and 
presently  crumbles.  Thus  we  see  how  a  chemical  change 
may  also  produce  physical  change  as  one  of  its  results. 

Other  chemical  changes  produce  similar  results.     Some 
of  the  substances  which  result  from  chemical  changes  in 


FIG.  40. — Talus  slope. 

the  rocks  are  thereby  rendered  soluble  in  water,  while 
other  minerals  are  soluble  without  chemical  change.  So 
we  see  that  the  combination  of  chemical  change  and  solu- 
tion is  one  of  the  most  important  of  the  causes  of  the 
breaking  down  or  disintegration  of  rock.  As  rock  breaks 
down,  soil  builds  up;  in  other  words,  rock  disintegration  is 
soil  formation. 

IV.  Organisms.  —  You  have  learned  that  plants  and  ani- 
mals contribute  directly  to  soil  formation,  their  dead  parts 
forming  humus  or  mould.  But  they  also  contribute  in- 
directly by  being  an  important  factor  in  this  great  process 


ORIGIN  OF  SOIL 


93 


of  "rock  weathering."    Their  disintegrating  effects  upon 

rocks  are  both  chemical  and  physical.     Roots  may  enter 

crevices  of  rock,  and  by  the  great  pressure  of  their  growth 

crack   these    rocks 

apart  (see  Fig.  41). 

Burrowing  animals 

produce  important 

effects  ;  their   bur- 

rows -increase   the 

aeration    (exposure 

to   air).     Earth- 

worms are  particu- 

larly important  in 

increasing   fertility 

of  soil  by  stirring 

it  up  and  improv- 

ing     its      aeration 

The  roots  of  plants 

excrete  substances  which  produce  corrosive  effects  upon 

rock,  especially  limestone. 

V.  Glaciers.  —  The  very  important  action  of  glaciers 
upon  rock  and  soil  has  already  been  described  (see  Chapter 
VIII). 


-  4I<  —  Showing  how  the  roots  of  trees  may  split 


QUESTIONS 

1.  What  is  soil  made  of? 

2.  What  is  our  greatest  mineral  resource? 

3.  What  is  humus? 

4.  Name  and  explain  the  different  factors  in  rock  weathering. 

5.  What  is  "talus"  and  how  is  it  formed? 

6.  How  does  water  act  on  limestone? 


CHAPTER  XII 
KINDS  *OF  SOIL 

Soil  may  be  classified  in  various  ways.  Thus  we  may 
classify  it  on  the  basis  of  its  origin,  that  is,  on  the  basis  of 
how  it  was  formed.  Or  we  may  classify  it  on  the  basis 
of  the  materials  which  principally  compose  it  and  give  it 
a  certain  character.  Both  of  these  methods  of  classi- 
fication are  important.  The  former  is  much  simpler,  but 
the  latter  is  more  important  when  it  comes  to  considering 
agricultural  possibilities. 

As  to  method  of  origin,  the  kinds  of  soil  are  compara- 
tively few,  for  the  ways  in  which  soil  is  formed  are  com- 
paratively few.  But  as  to  the  nature  of  its  contents,  the 
kinds  of  soil  are  very  many,  for,  as  you  have  seen,  soil  is 
a  mixture,  and  the  substances  which  enter  into  this  mix- 
ture are  numerous  and  their  proportions  are  various. 

You  have  learned  that  the  various  inorganic  substances 
of  which  soil  (and  rock)  is  composed  are  called  min- 
erals. Thus  clay  is  largely  composed  of  the  mineral  called 
feldspar,  while  sand  is  chiefly  made  up  of  quartz.  There 
are  many  different  kinds  of  minerals  and  each  kind  has  a 
definite  nature  of  its  own.  One  kind  may  have  a  very 
beneficial  effect  upon  the  fertility  of  soil,  while  another  kind, 
if  present  in  large  quantities,  may  have  an  injurious  effect; 
or  the  mineral  nature  of  the  soil  may  make  it  favorable  to 
the  growth  of  certain  kinds  of  plants,  but  unfavorable  to 
the  growth  of  other  kinds. 

94 


KINDS  OF  SOIL  95 

Thus  we  can  see  why  it  is  said  that  to  the  farmer  each 
field  has  a  problem  of  its  own.  The  fertility  of  a  field 
depends  largely  upon  the  physical  and  chemical  nature 
of  its  soil;  it  may  be  good  for  some  crops,  but  poor  for 
others;  it  may  be  good  for  a  certain  crop  one  season, 
but  poor  for  that  crop  the  next  season,  because  of  the 
changes  in  the  soil  which  that  crop  produces.  Many 
crops  gradually  "poison,"  or  reduce  the  fertility  of  soil 
for  crops  of  the  same  kind.  So  we  hear  of  what  is  called 
rotation  of  crops;  by  changing  his  crops  from  season  to 
season  the  farmer  really  changes  his  soil  and  so  helps  to 
keep  up  its  fertility.  The  changes  which  one  kind  of 
crop  makes  are  counterbalanced  by  the  changes  which 
another  kind  makes;  one  kind  will  succeed  another  kind 
better  than  it  will  succeed  itself.  This  is  because  the 
materials  which  one  kind  of  plant  takes  from  the  soil 
may  be  different  in  amount  from  those  taken  by  another 
kind,  and  the  changes  which  one  kind  produces  in  the 
soil  may  also  be  different  from  those  produced  by  another 
kind. 

Soil  as  to  Origin.  —  As  to  origin,  there  are  evidently  two 
great  kinds  of  soil.  There  is  the  kind  which  has  been 
formed  just  where  you  find  it,  and  there  is  the  kind  which 
was  formed  somewhere  else,  and  then  moved,  usually  by 
water,  to  the  place  where  you  find  it.  The  former  is  called 
residual  soil;  the  latter,  transported  soil. 

The  fertility  of  residual  soils  varies  more  than  the  fer- 
tility of  transported  soils;  some  residual  soils  are  fertile, 
others  are  sterile;  this  depends  chiefly  upon  the  nature 
of  the  parent  rocks  from  which  they  were  formed.  Trans- 
ported soils,  on  the  other  hand,  are  much  more  uniformly 


96  ELEMENTARY  SCIENCE 

fertile;  this  is  because  of  the  thorough  mixing  of  materials 
which  occurs  in  the  process  of  their  formation;  the  result 
is  usually  a  complex  mixture  which  is  very  good  for  the 
growth  of  plants.  Residual  soil  whose  parent  rock  was 
sandstone  is  sandy  and  generally  infertile.  Residual  clay 
comes  from  shale,  and  is  generally  more  fertile  than  very 
sandy  soil,  but  it  is  usually  heavy  and  difficult  to  till. 
(Tillage  means  the  working  of  soil  for  crops,  as  by  plough- 
ing, harrowing,  etc.)  Residual  soil  derived  from  limestone 
is  usually  fertile  and  tillable;  a  certain  amount  of  lime  in 
the  soil  is  very  desirable  for  plant  growth. 

Transported  soils  may  be  subclassified  on  the  basis  of 
the  transporting  agencies.  Thus  we  have  alluvial  soils, 
which  are  composed  of  materials  (sediment)  transported 
and  deposited  by  rivers.  These  soils  are  highly  fertile 
and  easily  tillable,  both  on  account  of  their  texture  and 
the  levelness  of  their  surface.  So  it  is  not  surprising  to 
find  that  agriculture  and  civilization  first  developed  near 
the  mouths  and  along  the  lower  stretches  of  great  rivers, 
such  as  the  Nile  in  Egypt  and  the  Hwang  Ho  in  China. 
For  along  these  ancient  rivers  there  are  great  areas  of 
alluvial  soil,  very  excellent  for  farming,  and  capable  of 
supporting  a  dense  population.  Such  rivers,  where  they 
flow  through  level  country,  develop  flood-plains,  that 
is,  plains  which  have  been  levelled  over  and  built  up  by 
the  wide  floods  of  the  river  (see  Fig.  42).  "Ancient  civiliza- 
tions were  confined  so  generally  to  rich  flood-plain  soils  that 
the  period  before  800  B.  C.  has  been  called  the  Fluvial 
Period"  (Salisbury).  In  the  United  States  the  rich  "bot- 
tom-lands" of  river- valleys  are  famous  for  their  fertility, 
and  are  of  high  value,  though  subject  to  the  risk  of  loss 
by  flood  of  fences,  houses,  and  livestock. 


KINDS  OF  SOIL  97 

Wind  has  played  and  continues  to  play  a  far  more  im- 
portant part  in  the  transportation  of  soil  than  one  is  likely 
to  think.  However,  if  you  recall  the  sand-storms  of  the 
desert  of  which  you  have  heard,  and  then  think  of  the 
enormous  stretches  of  time  through  which  this  agency 
has  been  at  work,  you  can  understand  how  great  results 


FIG.  42. — Flood-plains. 

have  been  accomplished.  Soils  composed  chiefly  of  sand 
or  dust  which  was  once  blown  where  they  are,  are  called 
eolian  soils. 

Sand-dunes  (see  Fig.  43)  are  formed  by  the  winds.  Of 
course,  while  the  sand  is  still  being  blown  about,  there  is 
little  chance  for  plants  to  gain  a  foothold.  But  gradually, 
on  dune-forming  shores,  the  beach  moves  seaward  or  lake- 
ward  as  the  sand  accumulates;  and  the  farther  a  dune 
gets  from  the  beach,  the  less  it  is  exposed  to  the  wind. 
So  plants  begin  to  cover  it,  their  roots  holding  it  together, 
and  presently  we  may  have  a  dense  vegetation  growing 


98  ELEMENTARY  SCIENCE 

upon  this  strictly  eolian  soil,  as  along  the  east  coast  of 
Lake  Michigan.  The  "sand-hill  region"  of  western  Ne- 
braska is  a  great  tract  which  is  practically  useless  for 
agriculture  because  of  the  strong  winds  which  blow  the 
loose  sand  about. 

Eolian  soil  that  contains  more  of  dust  than  of  sand  is 
called  loess  (pron.  lus).    Soil  which  is  partly  sand  and 


FIG.  43.— Sand-dunes. 

partly  clay  (and  usually  partly  humus)  is  called  loam.  So 
we  see  that  loess  is  eolian  loam.  Some  of  the  richest  agri- 
cultural regions  are  regions  of  loess.  There  is  much  of  loess 
in  the  Mississippi  and  Missouri  river-basins.  Loess  for- 
mations in  China  are  famous  for  their  depth,  and  many  of 
those  in  Europe  for  their  fertility. 

Ice  was  (and  to  some  extent  is  to-day)  a  third  great 
factor  in  the  transportation  of  soil.  You  have  read  of  the 
glaciers  and  of  the  great  effects  they  produced,  especially 
in  northeastern  and  north-central  United  States  (see 
chapter  VIII).  You  know  that  the  material  deposited  by 


KINDS  OF  SOIL  99 

glaciers  is  called  till  or  glacial  drift,  and  that  the  soils  af- 
fected by  glaciers  are  usually  quite  fertile. 

Soil  as  to  Nature.  —  The  classification  of  soils  as  to 
their  chemical  nature  is  much  too  complicated  a  subject 
to  take  up  in  a  book  of  this  kind.  But  we  should  note 
that  this  is  a  very  important  matter  in  scientific  farming. 
By  means  of  chemical  analysis  it  is  possible  to  discover 
in  what  mineral  and  organic  substances  a  given  soil  is 
rich  or  poor,  as  the  case  may  be.  The  substances  which 
crops  need  being  known,  it  is  evident  that  such  analyses 
give  the  farmer  a  scientific  basis  upon  which  to  determine 
what  kinds  and  amounts  of  fertilizers  should  be  applied 
to  his  fields.  Without  such  analyses,  the  use  of  fertilizers 
is  largely  a  matter  of  guesswork.  We  mean  by  fertilizer 
anything  used  to  improve  the  fertility  of  soil.  Lime, 
rock-phosphate,  bone-dust,  potash,  and  manure  are  exam- 
ples of  fertilizers;  all  of  these  contain  substances  which  are 
very  necessary  to  the  growth  of  plants. 

The  classification  of  soil  as  to  its  physical  nature  de- 
pends chiefly  upon  the  size  of  the  soil  grains.  Thus,  sand 
has  much  larger  grains  than  clay,  while  loam,  as  you 
have  noted,  is  a  mixture  of  the  two.  You  have  also 
learned  (see  page  83)  how  important  a  part  the  size  of 
soil  grains  plays  in  the  matter  of  water  retention  by  the 
soil.  Generally  speaking,  for  agricultural  purposes,  sand 
is  too  porous;  it  lets  the  water  escape  too  readily.  Clay 
is  too  dense;  it  holds  water  too  tenaciously  and  is  too 
difficult  to  work.  Loam  is  a  "happy  medium"  between 
the  two,  especially  if  its  richness  be  increased  by  a  quan- 
tity of  humus. 

Of  course  soils  vary  exceedingly  as  to  the  relative  pro- 


100 


ELEMENTARY  SCIENCE 


portions  of  sand  and  clay,  but  the  following  rough  classi- 
fication may  be  made: 


SAND 

CLAY 

Sandy  soil             

80% 

10% 

Sandy  loam            

60-70% 

10-25% 

Loam           ....             

40-60% 

I  "r-3O% 

Clay  loam           ...                     .    .          .... 

10-35% 

•*o-s;o% 

Clay  

10% 

60-00% 

Summary  :    Soils  may  be  classified  as  follows: 

As  to  origin.  —  Residual  soil. 

Transported  soil. 

Alluvial  soil. 

Eolian  soil. 

Glacial  soil. 

As  to  nature.  —  Chemically,   on   the  basis  of  chemical 

analyses  of  its  constituents,  or 
Physically,  on  the  basis  of  the  size  and 
texture  of  its  grains  (see  table  above). 

QUESTIONS 

1.  What  are  the  different  kinds  of  soils? 

2.  Why  do  we  "rotate"  crops? 

3.  Why  does  the  fertility  of  residual  soils  vary  more  than  the  fer- 

tility of  transported  soils? 

4.  In  what  respects  are  the  following  residual  soils  good,  and  in 

what  respects  are  they  bad:  sandy  soil,  day  soil,  limestone 
soil? 

5.  What  are  alluvial  soils?    Are  they  good  or  bad ? 

6.  What  are  eolian  soils? 

7.  What  are  the  various  things  that  transport  soil? 

8.  What  are  fertilizers?    How  do  we  learn  what  fertilizers  to  use? 

9.  What  is  the  best  kind  of  soil,  and  why? 


CHAPTER 
FERTILITY  AND  SOIL  LIFE:    BACTERIA 

Soil  Fertility  Dependent  on  Bacteria.  —  The  fertility  of 
soil  depends  upon  certain  things  which  you  have  already 
learned.  But  it  also  depends  upon  another  thing  which  we 
have  not  mentioned.  That  thing  is  the  hidden  life  within 
the  soil.  This  "  hidden  life  "  to  which  we  refer  is  not  the  life 
of  the  roots  of  plants  whose  stems  and  leaves  grow  up  into 
the  air  and  sunshine.  Nor  is  it  the  life  of  animals  which, 
like  earthworms,  burrow  in  the  soil,  though  they,  too,  affect 
soil  fertility.  It  is  life  which  is  hidden  not  only  because 
it  is  hi  the  soil,  but  because,  even  when  we  do  uncover  it, 
we  cannot  see  it.  The  individuals  which  compose  it  are 
too  small  to  be  seen  by  the  naked  eye.  They  are  micro- 
organisms (mikros,  small);  organisms  so  small  that  they 
can  be  seen  only  with  the  aid  of  a  microscope. 

These  micro-organisms  are  mostly  plants,  but  they  are 
not  green  plants,  and  their  life  habits  are  very  different 
from  those  of  green  plants.  They  are  not  capable,  as  green 
plants  in  sunlight  are,  of  making  their  own  food;  that  is, 
they  cannot  take  inorganic  materials  from  soil  and  air  and 
transform  them  into  organic  materials.  None  the  less, 
their  way  of  life  appears  to  be  very  successful,  for  they 
occur  in  teeming  millions.  Rich  soil  is  as  full  of  invisible 
life  as  is  stagnant  water.  Forest  soil,  especially,  supports 
within  itself  a  population  more  dense  than  the  population 
of  our  most  crowded  cities,  and  the  plants  which  grow  up 


102 


ELEMENTARY  SCIENCE 


from  such  soil  are  greatly  affected  by  the  hidden  life  which 
swarms  among  their  roots,  and  finds  its  nourishment  in 
rotting  branches  and  decaying  leaves. 

Chief  among  these  micro-organisms  of  the  soil  we  should 
note  the  bacteria,  which,  as  you  may  already  know,  are 

very  abundant  else- 
where as  well  as  in 
soil  (see  Fig.  44). 
Doubtless  you  have 
heard  that  bacteria 
cause  disease,  and 
perhaps  you  natu- 
rally think  of  them 
as  the  enemies  of 
mankind.  It  is 
true  that  many  dis- 
eases are  caused  by 
bacteria;  it  is  also 
true  that  most  of 
these  diseases 
might  be  prevented 
by  careful  living. 
And  yet  the  bene- 
fits which  bacteria 
confer  on  mankind 
are  quite  as  impor- 
tant as  the  harm 
they  do.  It  is  the 

green  plants  which  make  our  food;  but  it  is  the  bacteria, 
chiefly,  which  reduce  the  dead  parts  and  bodies  of  organ- 
isms to  a  condition  in  which  they  can  be  built  up  into  food 
again.  Just  as  we  depend  on  the  work  of  green  plants  for 


FIG.  44.— Various  kinds  of  bacteria. 


FERTILITY  AND   SOIL  LIFE  103 

food,  so  it  appears  that  green  plants  depend  upon  the  work 
of  soil  bacteria  for  materials  out  of  which  they  make  food. 
Thus  we  see  that  the  changing  of  materials  from  the  in- 
organic to  the  organic  condition  and  back  again  occurs 
in  a  sort  of  cycle.  At  the  beginning  of  this  cycle  we  have 
the  work  of  green  plants.  At  the  end  of  it  we  have  the 
work  of  bacteria  and  other  micro-organisms,  which  change 
the  complex  dead  organic  substances  into  simpler  sub- 
stances which  green  plants  can  use.  Green  plants  cannot 
use  these  materials  until  they  have  been  so  changed. 

Importance  of  Nitrogen  to  Plants.  —  Bacteria  increase 
the  amount  of  nitrogen  in  the  soil  which  is  available  for  the 
use  of  green  plants,  and  this  is  very  important.  By  avail- 
able nitrogen  we  mean  nitrogen  in  such  condition  that 
plants  can  use  it.  Nitrogen  is  necessary  to  plant  life. 
Four-fifths  of  the  air  is  nitrogen,  as  you  have  learned.  In 
the  air  then  there  seems  to  be  an  inexhaustible  supply. 
But  green  plants  cannot  use  this  atmospheric  nitrogen  at  all; 
they  cannot  use  it  any  more  than  we  can  use  sea-water  to 
drink.  Shipwrecked  sailors  might  have  a  whole  ocean  of 
water  about  them,  and  yet  perish  of  thirst  for  lack  of 
available  water  to  drink.  So  with  green  plants  and  nitro- 
gen; the  nitrogen  to  be  available  for  them  must  be  pres- 
ent in  certain  compounds  (nitrates)  which  are  very  differ- 
ent from  the  free  nitrogen  of  the  air. 

Certain  bacteria,  sometimes  called  the  nitrogen-fixing 
bacteria,  have  the  power  to  use  the  nitrogen  of  the  air,  and 
they  change  it  into  the  compounds  that  can  be  used  by 
plants.  This  is  not  the  only  method  by  which  such  com- 
pounds are  produced  in  nature,  but  it  is  a  very  important 
one. 


104 


ELEMENTARY  SCIENCE 


Farmers  realized  that  clover  improves  the  fertility  of 
soil  long  before  they  found  why  this  is  so;  later  it  was 
found  that  clover  enriches  the  soil  in  these  nitrogen  com- 
pounds that  are  used  by  plants.  The  reason  why  clover 
and  other  of  its  plant  relatives  produce  this  result  is  that 
"nitrogen-fixing"  bacteria  live  and  thrive  in  abundance 

on  the  roots  of  these  plants. 
They  live  in  little  swellings 
of  the  roots  called  nodules 
or  tubercles;  these  nodules 
are  produced  by  the  activ- 
ity of  the  bacteria  which 
live  inside  the  roots  (see 
Fig.  45).  There  is  air,  as 
you  know,  hi  the  soil,  and 
the  bacteria  in  these  nod- 
ules absorb  nitrogen  from 
this  air,  and  combine  it 
with  other  substances.  The 
clover  hay  is  cut,  and  the 
roots  left  to  decay  in  the 
soil,  thus  making  it  much 
more  fertile.  Note  that 
clover  and  bacteria  are  both  necessary  for  this  process. 
When  the  nitrogen-fixing  bacteria  are  not  abundant,  the 
clover  crop  is  much  poorer  than  when  they  are.  Some- 
times fields  are  artificially  "inoculated"  with  the  desired 
bacteria.  Soy-beans,  cow-peas,  svveet  clover,  and  alfalfa 
are  some  of  the  relatives  of  clover  which  also  bear  nitro- 
gen-fixing bacteria  and  root-tubercles  abundantly.  These 
plants  all  belong  to  the  same  family,  the  legume  or  pea 
family. 


FIG.  45.— Root-tubercles. 


FERTILITY  AND  SOIL  LIFE 


105 


You  have  learned  what  is  meant  by  "rotation  of  crops" 
(see  page  95).  To  grow  corn  or  wheat  for  two  years  and 
then  clover  for  one  is  a  common  example  of  rotation. 
Various  benefits  arise  from  such  rotation,  but  probably  the 
principal  one  is  the  enrichment  of  soil  in  nitrogen  in  the 
way  indicated.  Soy-beans  are  sometimes  grown  by 
farmers  and  then  "ploughed  under,"  the  sole  value  of  this 
crop  being  to  enrich  the  soil  in  nitrogen  for  other  crops. 


FIG.  46. — One  of  the  fungi. 

Besides  the  bacteria,  which  are  the  smallest  of  the 
micro-organisms,  there  are  hosts  of  other  kinds  of  lowly 
organisms  which  live  in  rich  soil  and  affect  its  fertility. 
There  are,  for  example,  many  kinds  of  colorless  plants 
(fungi}  which  grow  underground  in  the  form  of  spreading 
threads,  more  delicate  even  than  the  threads  of  cobweb  (see 
Fig.  46).  Occasionally,  when  the  conditions  are  just  right, 
a  mass  of  such  cobwebby  threads  will  produce  a  structure 
much  larger  than  itself  which  grows  up  above  the  surface; 
these  structures  contain  millions  of  almost  invisible,  pow- 


io6 


ELEMENTARY  SCIENCE 


der-like  spores,  which  reproduce  the  plant.  Toadstools, 
mushrooms,  and  puffballs  are  examples  of  such  structures 
(see  Fig.  47). 

The  effects  which  such  plants  produce  upon  the  fer- 
tility of  soil  are  not  well  understood,  but  it  is  known  that 
their  threads  penetrate  everywhere  in  soil  that  contains 
organic  matter,  and  they  certainly  produce  many  chemical 
changes  in  such  soil.  Some  of  them  form  felt-like  coat- 


FIG.  47.— Puffballs. 

ings  on  the  tips  of  the  roots  of  certain  plants,  and  seem 
to  be  of  benefit  to  such  plants  (see  Fig.  48). 

Enough  has  been  said  to  prove  to  you  that  soil  is  a  very 
complex  thing.  Except  where  it  is  quite  bare  of  plants, 
like  bare  sand,  it  contains  more  or  less  of  organic  material, 
and  this  organic  material  furnishes  food  for  hosts  of  micro- 
organisms. These  micro-organisms,  along  with  the  physical 
and  chemical  nature  of  the  soil,  and  its  relations  to  water, 
air,  and  heat  are  all  factors  in  determining  fertility  or 
infertility. 


FERTILITY  AND  SOIL  LIFE  107 

Fertilizers.  —  Formerly  it  was  thought  that  when  fer- 
tilizers were  added  to  soil*  the  plants  simply  used  the 
fertilizers  directly  as  "food."  But  now  we  have  found 
that  fertilizers  may  produce  their  good  effects  without 
acting  as  food  at  all.  They  may  cause  the  destruction 
of  poisonous  substances  which  are  in  the  soil,  or  they  may 
stimulate  the  growth  of  micro-organisms  which  are  essen- 
tial to  its  fertility,  or  they  may  simply  change  the  chem- 
ical condition  of  the  soil  in  a  way  that  is  favorable  to  plants. 
Thus  fertilizers  benefit  the  soil  in  various  ways,  and  prob- 
ably will  be  used  even  more  in  the  future 
than  in  the  past.  In  the  future,  how- 
ever, we  may  expect  that  the  exact  ef- 
fects which  they  produce  will  be  better 
understood,  and  this  will  lead  to  a  more 
scientific  and  more  economical  use  of 
them. 

Soil     Fertility    and    Food-Supply.  —  ^l 

The  whole  question  of  soil  fertility  is  of  FlG-  £|~ 
huge  importance  to  every  one  of  us;  the 
future  welfare  of  the  human  race  appears  to  depend  upon 
it  more  than  upon  any  other  single  thing.  As  the  years 
have  gone  by,  the  fertility  of  many  of  our  greatest  agricul- 
tural regions,  especially  in  the  United  States,  has  been 
decreasing;  the  amount  of  crop  per  acre  has  been  getting 
less.  Meanwhile  each  year  the  population  of  the  world 
increases.  Each  year  there  are  more  mouths  to  be  fed, 
more  hunger  to  be  satisfied.  Are  we  sure  that  each  year 
there  will  be  more  food?  Most  of  the  fertile  land  in  the 
world  is  already  under  cultivation,  and  if  fertility  continues 
to  decrease  while  food-consumption  continues  to  increase, 


io8  ELEMENTARY  SCIENCE 

the  outcome  is  evidently  going  to  be  very  bad  for  the  human 
race. 

However,  there  is  no  real  danger  if  the  farmers  will  use 
better  methods  of  agriculture.  Science  has  shown  how  to 
prevent  loss  of  the  fertility  of  the  soil,  and  scientists  con- 
tinue each  year  to  make  new  discoveries  which  are  of  the 
greatest  importance  to  agriculture.  They  have  proved 
how  certain  fertilizers  and  certain  methods  of  farming 
increase  the  crops,  and  they  have  found  and  developed  new 
races  of  plants  which  will  yet  further  increase  the  food 
production  of  the  world.  But  the  difficulty  is  to  get  the 
farmers  to  heed  the  facts  of  science.  Usually  they  do  not 
like  to  take  the  trouble  to  change  their  methods,  and  gen- 
erally they  stick  to  the  old  wasteful  ways  even  in  the  face 
of  positive  evidence  that  other  ways  are  better.  So  our 
hope  lies  chiefly  in  the  next  generation,  of  which  you  are 
a  member.  It  is  to  be  hoped  that  your  training  in  science 
will  be  thorough  enough  to  make  you  fully  appreciate  all 
that  science  offers  of  improvements,  both  in  agriculture 
and  in  other  great  human  activities.  Whether  you  will 
heed  what  science  teaches  or  not,  that  is  another  matter, 
a  matter  for  you  yourself  to  determine. 

QUESTIONS 

1.  What  are  bacteria? 

2.  Why  are  bacteria  useful  in  the  soil? 

3.  Can  green  plants  use  the  nitrogen  of  the  air?     How  is  nitrogen 

made  available  for  them? 

4.  Why  does  clover  improve  the  fertility  of  the  soil  ? 

5.  Why  do  we  rotate  crops? 

6.  What  are  fungi,  and  what  good  are  they  in  the  soil? 

7.  What  things  do  fertilizers  do? 


CHAPTER  XIV 
MECHANICAL  WORK:  SIMPLE  MACHINES 

Work  may  be  classified  into  that  which  is  mechanical 
and  that  which  is  mental.  Man's  work  is,  or  should  be, 
a  combination  of  both.  A  man  is  generally  paid  for  his 
work  in  proportion  to  the  amount  of  brains  he  puts  into 
it.  That  is,  mental  work  is  usually  more  highly  paid 
than  mechanical  work.  A  "day-laborer"  is  usually  ex- 
pected to  do  only  mechanical  work,  which  any  healthy 
and  sane  man  can  perform.  Brain-workers,  however, 
are  not  so  common.  Those  who  can  do  really  high-grade 
mental  work  are  comparatively  few,  and  command  high 
prices,  if  there  be  a  demand  for  their  kind  of  work. 

There  has  always  been  plenty  of  work  for  man  and  there 
always  will  be.  When  we  hear  of  scarcity  of  work  it  really 
means  scarcity  of  money  to  pay  for  it.  There  is  no  danger 
of  the  work  itself  giving  out.  The  kinds  of  work  to  be  done 
will  change,  but  work  itself  we  shall  have  always  with  us. 
Though  labor-saving  inventions  enable  one  man  to  do 
work  formerly  requiring  many,  new  needs  for  work  almost 
keep  pace  with  the  inventions.  But  this  important 
thing  should  be  noted.  As  the  world  grows  older,  it  is 
the  opportunities  for  brain-work  which  are  increasing, 
while  the  opportunities  for  mechanical  work  are  decreas- 
ing. It  is  easy  to  see  that  machines  may  presently  do 
nearly  all  the  purely  mechanical  work,  and  to  be  sure  of 
profitable  work,  one  must  learn  to  do  things  which  require 
skill  and  brains. 

109 


HO  ELEMENTARY  SCIENCE 

So  it  is  equally  clear  that,  as  the  world  grows  older, 
the  need  of  its  young  people  for  education  becomes 
greater.  The  education  which  was  good  enough  for  your 
grandfather  may  not  be  good  enough  for  you.  Times 
have  changed.  You  must  stay  in  school  as  long  as  you 
can  to  make  sure  of  earning  a  living.  The  modern 
world  into  which  you  are  going  demands,  above  all 
things,  minds  which  are  trained  and  well-informed.  Mere 
strength  and  smartness  count  for  little  in  the  world's 
work  of  to-day,  and  they  will  count  for  less  and  less  as 
time  goes  on. 

The  progress  of  man  may  be  measured  by  the  extent  to 
which  he  has  put  brains  into  his  work.  Indeed  the  prog- 
ress of  man  may  be  defined  as  the  process  of  putting 
brains  into  work.  It  is  the  process  of  thinking  in  con- 
nection with  doing,  the  process  of  making  behavior  a  ful- 
filment of  careful  thought.  This  defines  the  progress  of 
the  individual  as  well  as  of  the  race.  It  is  a  goal  toward 
which  we  all  strive. 

Man's  first  use  of  labor-saving  devices  was  probably 
not  reasoned  out.  It  was  more  or  less  instinctive.  He 
felt  his  way,  rather  than  thought  his  way  to  the  doing  of 
certain  things  in  certain  ways.  He  learned  by  experience 
rather  than  by  reason.  Thus  the  first  primitive  men  who 
pried  up  large  stones  with  sticks,  or  threw  small  ones  hi 
slings,  probably  did  these  things  without  thinking  much 
about  them.  There  is  a  scientific  explanation  as  to  why 
it  is  easier  to  pry  up  a  stone  than  to  lift  it,  but  a  small 
boy  will  do  it  without  having  any  idea  of  the  explanation. 
Similarly  such  things  were  done  in  the  childhood  of  the 
race.  The  explanations  came  later.  These  explanations 
were  the  beginnings  of  science. 


MECHANICAL  WORK  in 

Mechanical  Work.  —  The  work  which  this  chapter  dis- 
cusses is  mechanical  work.  It  is  the  kind  of  work  which 
can  be  measured  in  foot-pounds  or  horse-power.  It  is  the 
kind  of  work  done  for  us  by  animals  and  by  machinery. 

An  ancient  legend  illustrates  the  principle  which  man 
has  chiefly  used  in  his  simplest  and  oldest  labor-saving 
devices.  A  king  had  six  sons.  He  decided  to  bestow  his 
kingdom  upon  the  one  having  the  most  intelligence.  To 
test  this  he  gave  to  each  a  bundle  of  sticks,  and  asked 
them  to  see  who  could  break  his  bundle  in  two.  Five  of 
them  tugged  and  strained  and  did  their  best  to  break  the 
bundles  by  main  strength.  But  the  sixth  untied  his 
bundle  and  broke  the  sticks  one  by  one.  On  him  the  father 
bestowed  his  kingdom. 

This  simple  principle  of  doing  things  gradually  explains 
many  of  our  simple  machines.  Let  us  see  how  it  applies. 
Why  is  it  easier  to  climb  a  steep  hill  by  a  crooked  path 
than  by  a  straight  one?  Why  is  it  easier  to  roll  a  heavy 
barrel  up  a  tilted  board  than  it  is  to  lift  it  straight  up  to 
the  same  height?  Both  of  these  are  cases  of  doing  things 
gradually,  cases  in  which  the  principle  used  is  called  the 
principle  of  the  inclined  plane.  If  a  barrel  weighing  three 
hundred  pounds  is  to  be  lifted  to  a  height  of  one  foot, 
three  hundred  foot-pounds  of  energy  must  be  used.  If  it 
is  lifted  straight  up  all  this  energy  must  be  applied  at 
once.  But  if  it  is  pushed  up  an  inclined  plane  three  feet 
in  length,  and  the  pushing  power  is  applied  parallel  to  the 
plane,  then  only  one  hundred  foot-pounds  need  be  applied 
at  once.  In  other  words,  we  have  to  move  the  barrel 
three  times  as  far,  but,  as  compared  with  lifting  it  up 
straight,  we  have  to  work  only  one-third  as  hard  while 
we  are  doing  it. 


H2  ELEMENTARY  SCIENCE 

Take  the  case  of  winding  up  a  well-bucket  on  a  wind- 
lass (see  page  55).  Suppose  the  circumference  of  the  cir- 
cle described  by  the  handle  of  the  windlass  when  it  is 
turned  is  five  times  as  great  as  the  circumference  of  the 
axle  on  which  the  rope  is  wound.  Then,  for  each  foot 
that  the  bucket  ascends,  your  hands  pushing  the  handle 
must  move  five  feet.  In  other  words,  the  work  of  lifting 
the  bucket  one  foot  is  distributed  through  a  five-foot 
effort,  rather  than  concentrated  in  a  one-foot  effort,  as  it 
would  be  if  you  pulled  up  straight.  So  it  is  just  five  times 
as  easy  to  push  the  handle  one  foot  as  it  would  be  to  pull 
the  bucket  up  one  foot.  It  is  just  like  breaking  five  sticks 
one  at  a  time  instead  of  trying  to  break  them  all  at  once. 
What  you  may  not  be  able  to  accomplish  by  one  great 
effort,  you  can  accomplish  by  a  continuation  of  small  effort, 
and  this  in  the  end  accomplishes  as  much  as  some  stronger 
person  might  be  able  to  accomplish  all  at  once.  The 
amount  of  work  done  is  the  same  whether  it  be  done  all  at 
once  or  gradually,  with  the  great  advantage  in  favor  of 
the  gradual  method  that  it  enables  us  to  do  things  which 
by  direct  methods  we  cannot  do  at  all. 

Take  the  case  of  prying  up  a  heavy  stone  with  a  stick 
or  a  crowbar.  In  order  to  get  a  purchase  you  place  a  piece 
of  wood  or  stone  under  the  crowbar,  near  the  thing  to  be 
pried  up.  The  crowbar  is  a  lever.  The  thing  that  gives 
the  purchase  is  the  fulcrum.  Now  suppose  it  is  one  foot 
from  the  fulcrum  to  your  hands,  and  four  inches  from  the 
fulcrum  to  the  thing  you  are  prying  up.  Then  for  every 
inch  that  the  lower  end  of.  the  bar  rises,  your  hands  at  the 
other  end  must  push  down  three  inches.  If  you  put  all 
your  strength  and  weight  on  the  end  of  the  bar,  and  move 
it  down  three  inches,  that  strength  and  weight  is  multi- 


MECHANICAL  WORK 


plied  three  times  as  applied  to  lifting  the  heavy  stone 
one  inch.     Do  you  see  that  the  same  principle  is  involved  ? 

Take  the  case  of  a  pulley  (see  Fig.  49).  r__i 

The  object  to  be  lifted  is  held  up  by  two 
lengths  of  rope,  one  of  these  being  fastened 
to  the  hook  under  the  upper  pulley,  the 
other  passing  over  it  and  down  to  your 
hands.  Now  do  you  see  that  to  lift  that 
object  one  foot  you  must  pull  the  rope  two 
feet?  This  means  that  your  effort  as  ap- 
plied to  lifting  the  weight  will  be  doubled, 
though  it  will  operate  through  only  half  the 
distance  that  you  pull  the  rope.  By  arrang- 
ing a  third  pulley,  the  strength  of  the  pull 
on  the  weight 
may  be  trebled, 
the  distance 
through  which 
the  pull  must  be 
made  being,  of  course,  also 
trebled.  You  may  have  seen 
houses  moved  by  a  combination 
of  windlass  and  pulleys,  and  you 
must  have  noted  that  the  horses 
turning  the  windlass  move  much 
faster  than  the  house  which  they 
are  moving.  It  is  the  same  prin- 
ciple at  work.  The  more  you 
increase,  by  extra  pulleys,  the 
power  applied  to  the  object  to  be  moved,  the  longer  must 
be  the  pulling  done  at  the  other  end  (see  Fig.  50). 
This  principle  may  be  expressed  mathematically.  The 


FIG.  49. — The  sim- 
ple pulley. 


FIG.  50. — Compound  pulleys. 


ELEMENTARY  SCIENCE 


TTPT 


power  expended  multiplied  by  the  distance  through  which 
it  is  expended  is  equal  to  the  weight  moved  multiplied  by 
the  distance  through  which  it  is  moved.  In  other  words, 
the  work  done  is  equal  to  the  force  expended  multiplied 

by  the  distance  through 
which  that  force  acts. 

A  lever,  of  which  you 
have  seen  the  crowbar  to 
be  an  example,  is  evi- 

FIG.  51. — Principle  of  the  scales.  r     ' 

dently  a  device  which 

increases  power  at  the  expense  of  speed,  which  is  another 
way  of  saying  that  the  job  is  done  gradually.  Evidently, 
also,  three  things  are  essential  to  a  lever;  namely,  the 
point  at  which  the  power  is  applied,  the  point  at  which 
it  is  exerted,  and  the  point  of  purchase  (the  fulcrum). 
These  points  may  be  variously 
arranged  with  reference  to  each 
other,  and  on  this  basis  levers  are 
divided  into  three  classes. 

A  lever  of  the  first  class  is  one 
in  which  the  fukrum  is  between  the 
applied  power  and  the  resistance. 
The  crowbar  is  a  lever  of  this  type. 
So  is  a  pair  of  scales  on  which  your 
weight  is  counterbalanced  by  a 
much  smaller  weight  at  the  end  of 
a  bar.  Another  weight  slides  to 

and  fro  on  the  bar.  The  heavier  you  are,  the  more  weight 
it  takes  to  bring  down  the  bar  (see  Figs.  51  and  52). 

In  a  lever  of  the  second  class  the  resistance  is  between  the 
applied  power  and  the  fulcrum  (see  Fig.  53).  This  is  well 
illustrated  by  the  nutcracker,  which  also  well  illustrates 


FIG.  52. — A  lever  of  the  first 
class. 


MECHANICAL  WORK  115 

the  gradual  application  of  power.  You  apply  power 
gradually  to  a  nutcracker  so  that  the  shells  will  not  fly  on 
the  floor  and  the  kernels  be  crushed.  A  wheelbarrow  is 
another  lever  of  this  class. 

In  levers  of  the  third  class  it  is  the  applied  power  which 
is  between  the  fulcrum  and  the  resistance  (see  Fig.  54). 


FIG.  53.— A  lever  of  the  second          FIG.  54.— A  lever  of  the  third 
class.  class. 

This  class  of  lever  you  can  illustrate  by  straightening  out 
your  arm  and  then  raising  your  hand  to  your  shoulder. 
Your  elbow  is  the  fulcrum,  and  the  weight  of  your  forearm 
is  the  resistance.  The  power  is  applied  at  that  point  in 
front  of  the  elbow  at  which  the  tendons  of  the  muscle  of 
your  upper  arm  (biceps)  are  attached  to  a  bone  of  the  fore- 
arm. You  lift  the  forearm  by  contracting  the  upper  arm, 
but  the  power  is  applied  just  in  front  of  the  elbow. 

QUESTIONS 

1.  Explain  the  principle  of  the  inclined  plane. 

2.  Why  is  it  easier  to  lift  a  thing  with  a  windlass  than  by  hand? 

3.  If  a  two-hundred-and-forty-pound  stone  were  on  the  short  end 

of  a  lever,  eight  inches  from  the  fulcrum,  and  if  the  long  end 


u6  ELEMENTARY  SCIENCE 

of  the  lever  were  four  feet  from  the  fulcrum,  how  much 
strength  would  it  take  to  lift  the  stone  by  means  of  the 
lever? 

4.  Explain  the  pulley. 

5.  Explain  the  difference  between  levers  of  the  first,  second,  and 

third  classes. 

6.  A  ton  of  coal  must  be  raised  twelve  feet.     The  force  which  can 

be  applied  is  one  hundred  and  fifty  pounds.  What  is  the 
shortest  inclined  plane  which  can  be  used  for  this  purpose? 

7.  In  what  class  of  levers  do  the  following  belong:  the  drawbridge, 

the  teeter-totter,  the  spring-board,  the  can-opener? 


CHAPTER  XV 
WORK  AND  ENERGY:   GRAVITATION 

Energy  Defined.  —  Behind  work  lies  energy.  Work  is 
what  is  done.  Energy  is  what  does  it.  Energy  is  that  by 
means  of  which  anything  moves  or  changes  other  things. 

The  word  force  has  a  meaning  similar  to  that  of  energy, 
but  there  is  a  difference  you  should  note.  Energy  is  a 
more  general  word  than  force.  By  force  we  usually  mean 
a  specific  kind  of  energy.  Thus  we  speak  of  the  force  of 
gravity  rather  than  of  the  energy  of  gravity.  Energy  is 
a  quality  which  all  forces  possess.  Forces  differ  in  their 
nature.  Energy  is  the  quality  which  is  common  to  them 
all. 

Forms  of  Energy.  —  Doubtless  you  have  heard  the  ex- 
pression "forms  of  energy."  Heat,  light,  electricity,  and 
the  fall  of  bodies  are  manifestations  of  different  forms  of 
energy.  We  call  them  forms  of  energy,  and  we  find  that 
they  can  be  transformed  into  each  other.  This  you  have 
learned,  as  in  the  case  of  transforming  water-power  into 
electricity  (see  page  70).  You  have  also  learned  of  the 
law  of  conservation  of  energy  (see  page  71).  This  law 
may  also  be  stated  as  follows:  no  form  of  energy  can  be 
increased  or  diminished  except  when  some  other  form  of 
energy  is  diminished  or  increased.  The  same  quantity  of 
energy  is  present  in  the  world  to-day  as  existed  ages 
ago.  You  cannot  increase  this  total  amount  of  energy  and 
you  cannot  take  from  it.  Whenever  energy  is  expended 
117 


n8  ELEMENTARY  SCIENCE 

it  is  either  transmitted  to  some  other  body  or  transformed 
into  some  other  form. 

Take  the  case  of  the  windlass  and  the  well-bucket  (see 
Page  55)-  Suppose,  by  winding,  you  have  raised  the 
full  bucket  up  to  the  axle  on  which  the  rope  is  wound. 
The  energy  you  expended  hi  pushing  the  handle  of  the 
windlass  is  now  transmitted  to  the  full  bucket  at  the  top 
of  the  well.  The  elevated  position  of  the  water  in  that 
bucket,  as  compared  with  the  water  in  the  well  from  which 
it  came,  is  an  investment  of  energy.  All  you  have  to  do  to 
prove  this  is  to  drop  the  bucket.  What  happens  ?  Down 
it  plunges,  and  round  and  round  the  handle  of  the  windlass 
spins,  far  faster  than  you  turned  it  in  the  opposite  direc- 
tion. Thus  the  energy  which  was  transmitted  to  the 
full  bucket  by  the  winding  of  the  handle  is  rapidly  trans- 
mitted back  again  whence  it  came.  Part  of  it  is  expended 
in  the  force  by  which  the  axle  unwinds,  and  part  of  it  in 
the  force  with  which  the  bucket  hits  the  surface  of  the 
water  in  the  well. 

Two  Kinds  of  Energy.  —  Here  evidently  are  two  kinds 
of  energy.  The  full  well-bucket  at  the  edge  of  the  curb 
had  energy  because  of  its  elevated  position.  The  handle, 
when  you  were  turning  it,  had  energy  because  of  its  motion. 
Energy  possessed  by  a  body  because  of  its  position  or 
condition  is  called  latent  or  potential  energy.  Energy 
possessed  by  a  body  because  of  its  motion  is  called  kinetic 
energy.  The  former  may  be  readily  changed  to  the  latter, 
as  when  you  dropped  the  bucket  from  the  brim  of  the  well. 
A  brick  at  the  top  of  a  building  possesses  an  amount  of 
potential  energy  directly  proportional  to  its  weight  and 
its  height  above  the  surface  of  the  earth.  If  the  brick 


GRAVITATION  119 

be  shoved  off,  this  energy  at  once  becomes  kinetic.  If  it 
falls  in  the  mud,  it  expresses  itself  in  the  depth  of  the  hole 
made  in  the  mud.  If  it  hits  some  one  on  the  head,  the 
pain  it  gives  him  will  be  a  rough  measure  of  its  energy. 
So  we  see  that  energy  when  merely  potential  accomplishes 
nothing.  It  may  lie  unexpressed  for  ages.  But  once  it 
becomes  kinetic,  something  happens.  Kinetic  energy  is 
the  energy  that  does  things. 

The  kind  of  energy  you  have  just  been  considering  is 
due  to  the  operation  of  the  law  of  gravity.  You  know  that 
energy  may  be  also  due  to  the  operation  of  the  laws  of 
heat,  or  of  light,  or  of  electricity,  but,  whatever  the  law 
of  nature  involved,  or  the  form  of  energy  under  considera- 
tion, the  principle  of  kinetic  and  potential  energy  always 
applies.  Thus  in  coal  there  is  much  potential  energy, 
irrespective  of  its  position.  This  energy  is  due  to  condition. 
It  expresses  itself  in  heat  when  we  burn  the  coal.  Heat 
is  a  kinetic  form  of  energy.  It  comes  from  the  coal,  be- 
cause the  molecules  of  coal  are  in  such  condition  that, 
when  burning,  they  unite  rapidly  with  oxygen,  and  give  off 
heat  in  connection  with  this  process. 

Gravitation.  —  Later  we  shall  discuss  those  manifesta- 
tions of  energy  known  as  heat,  light,  and  electricity.  But 
let  us  now  consider  the  law  of  gravitation.  We  have  al- 
ready considered  many  manifestations  of  energy  which 
are  due  to  its  operation.  One  of  these  is  the  fact  that 
water  runs  down-hill.  This  is  one  of  the  first  great  facts 
whose  effects  you  were  asked  to  consider.  You  have 
noted  its  e/ects.  Now  let  us  note  its  cause. 

Whether  it  be  the  fall  of  water  or  the  fall  of  an  apple, 
the  same  principle  is  at  work.  As  you  may  have  heard, 


120  ELEMENTARY  SCIENCE 

it  was  the  fall  of  an  apple  which  set  that  great  scientist 
Sir  Isaac  Newton  (1642-1727)  to  pondering  on  this  prob- 
lem; the  problem  of  why  the  apple  falls.  The  great  con- 
clusion which  he  reached  is  known  as  Newton's  law  of  uni- 
versal gravitation.  It  may  be  stated  as  follows: 

Every  mass  of  matter  in  the  universe  attracts  every  other 
mass  with  a  force  which  is  directly  proportional  to  the  product 
of  these  masses,  and  is  inversely  proportional  to  the  square 
of  the  distance  between  their  centers. 

According  to  this,  you  see,  the  apple  attracts  the  earth 
as  well  as  the  earth  attracts  the  apple.  But  the  apple  is 
so  very,  very  small  as  compared  with  the  earth  that  its 
effect  is  not  noticed. 

It  is  surprising  how  many  things  the  law  of  gravitation 
explains.  You  have  learned  that  the  tides  are  explained 
by  the  mass  attraction  or  gravitation  of  the  moon  acting 
on  the  water  of  the  seas.  But  a  far  greater  thing  is  the 
fact  that  the  paths  of  the  heavenly  bodies  are  explained 
by  gravitation.  You  have  heard  of  the  "stars  in  their 
orbits."  By  orbit  is  meant  the  path  or  course  which  is 
determined  by  effects  of  gravitation,  or  "pull"  of  heav- 
enly bodies  upon  one  another.  The  moon  is  held  in  its 
orbit  by  the  mass  attraction  of  the  earth,  and  the  planets 
(of  which  earth  is  one)  are  held  to  their  orbits  around 
the  sun  by  the  attraction  which  its  mass  exerts  upon 
them.  So,  also,  the  stars  in  their  courses  are  believed 
to  be  suns  around  which  other  planets  wheel  in  response 
to  the  same  law. 

Weight.  —  But  to  come  back  to  earth.  We  can  see  now 
that  the  weight  of  any  object  is  nothing  more  than  the  effect 
upon  it  of  the  law  of  gravitation.  Weight  is  an  expres- 


GRAVITATION  12i 

sion  of  the  tendency  to  be  drawn  to  the  center  of  the  earth. 
Every  particle  of  matter  has  this  tendency.  The  denser 
an  object  is,  the  greater  its  number  of  molecules  and, 
hence,  the  greater  its  weight. 

At  the  poles  of  the  earth,  the  distance  to  the  center  of 
the  earth  is  less  than  at  the  equator.  Hence,  an  object, 
according  to  Newton's  law,  at  one  of  the  poles  should 
weigh  less  than  the  same  object  at  the  equator.  Actual 
measurements  show  this  to  be  the  case. 

We  must  not  forget  that  this  law  of  mass  attraction 
works  both  ways.  That  is,  the  smaller  object  attracts 
the  greater  as  well  as  the  greater  the  smaller,  though  the 
only  visible  effect  may  be  the  effect  on  the  smaller  object. 
The  level  of  the  ocean  along  the  west  coast  of  South  America 
has  been  found  to  be  higher  than  its  level  at  other  places. 
This  is  believed  to  be  due  to  the  mass  attraction  of  the 
huge  range  of  the  Andes  Mountains.  The  attraction 
of  the  earth  as  a  whole  tends  to  keep  the  oceans  at  the 
same  level,  but  the  mass  of  the  Andes  above  the  water- 
level  is  sufficiently  great  to  cause  in  its  neighborhood  a 
measurable  variation  of  that  level. 

Why  does  oil  ascend  the  wick  of  a  candle?  This  is 
due,  at  least  in  part,  to  the  effect  of  the  mass  attraction 
of  the  wick  upon  the  molecules  of  oil. 

Note  that  the  explanation  of  facts  by  the  law  of  gravita- 
tion is  a  very  different  thing  from  explaining  gravitation. 
That  is  a  thing  which  no  one  has  explained.  This  is  one 
of  those  great  laws  of  nature  which,  in  elementary  work 
at  least,  we  do  not  attempt  to  explain.  We  simply  call 
these  laws  causes  and  seek  to  see  the  connection  between 
such  causes  and  their  e/ects.  It  is  very  important  to 
realize  that  given  causes  do  inevitably  produce  certain 


122  ELEMENTARY  SCIENCE 

effects.    But  to  attempt  to  explain  the  cause  of  the  causes 
is  philosophy  rather  than  science. 

QUESTIONS 

1.  What  is  the  difference  between  potential  energy  and  kinetic 

energy? 

2.  What  kinetic  energy  and  what  potential  energy  are  involved 

in  a  watch?    a  cannon?    a  dynamo?    a  piece  of  coal? 

3.  State  Newton's  Law  of  Universal  Gravitation. 

4.  Why  does  the  earth  move  around  the  sun,  instead  of  flying  off 

into  space? 

5.  Would  a  piece  of  lead  weigh  the  same  at  the  top  of  Pike's  Peak 

as  at  sea-level? 


CHAPTER  XVI 
INTRODUCTORY  AS  TO  HEAT:    DISCOVERY  OF  FIRE 

What  Is  Heat?  —  We  know  how  to  measure  heat 
and  we  know  how  to  feel  it.  But  what  is  it? 

It  is  easy  to  prove  that  our  feeling  of  heat  is  not  a  scien- 
tific means  of  measuring  it.  What  seems  hot  to  one  per- 
son may  seem  cool  to  another.  If  you  come  into  a  room 
after  a  brisk  walk  on  a  winter  day,  it  may  seem  uncom- 
fortably warm  to  you,  while  it  may  seem  uncomfortably 
cool  to  a  person  who  has  been  in  it  for  some  time.  If 
you  place  one  hand  in  hot  water  and  the  other  in  cold, 
and  then  transfer  both  to  lukewarm  water,  the  lukewarm 
water  will  feel  cool  to  the  hand  that  has  been  in  hot  water. 
After  you  have  been  eating  ice-cream,  does  ice-water 
"taste"  as  cold  as  it  did  before?  A  piece  of  wood  and  a 
piece  of  marble  may  be  of  just  the  same  temperature,  and 
yet  the  marble  will/ee/  much  cooler  than  the  wood.  Why 
is  this? 

A  little  of  this  sort  of  thing  is  quite  enough  to  prove  that 
the  feelings  of  a  human  being  for  heat  and  cold  are  not  a 
safe  guide  to  the  actual  facts  as  to  heat  and  cold.  One  of 
the  characteristics  of  science  is  that  it  deals  with  facts 
rather  than  feelings.  Therefore  scientific  instruments  are 
needed  for  recording  precisely  the  facts  as  to  heat  and  cold. 
With  one  of  these,  the  thermometer,  you  are  already  familiar. 
It  records  the  intensity  or  degree  of  heat.  The  degree  of 
heat  is  called  temperature,  so,  to  speak  exactly,  the  ther- 
123 


124  ELEMENTARY  SCIENCE 

mometer  is  a  measure  of  temperature  rather  than  of  heat. 
The  amount  of  heat  in  a  substance  is  another  thing.  That 
may  be  measured  by  another  instrument,  the  calorimeter, 
of  which  more  later. 

The  thermometer  was  invented  by  Galileo,  in  1593, 
one  hundred  and  one  years  after  the  discovery  of  America. 
Before  that  there  was  no  science  of  heat;  men  judged  of 
it  solely  by  their  feelings.  When  men  began  to  study  heat 
with  the  aid  of  the  thermometer  they  discovered  a  number 
of  things  about  it  which  they  could  not  discover  by  their 
feeling  alone.  Thus  the  thermometer  shows  that  the 
temperature  of  boiling  water  does  not  increase  however 
long  it  continues  to  boil,  a  thing  that  was  not  discovered 
when  the  temperature  of  objects  was  judged  by  fingers 
instead  of  thermometers.  Thermometers  also  show  that, 
when  heat  is  applied,  the  temperature  of  some  substances 
rises  much  more  rapidly  than  the  temperature  of  others. 
Thus  mercury  is  thirty  times  easier  to  heat  than  water; 
it  requires  only  one-thirtieth  as  much  heat  to  raise  the 
temperature  of  a  given  quantity  of  mercury  one  degree 
as  it  requires  to  raise  the  same  amount  of  water  one 
degree. 

Such  facts  gave  the  early  scientists  much  more  light  on 
the  subject  of  heat  than  they  had  had  before.  Their 
theories  as  to  heat  had  to  be  revised  in  the  light  of  new 
evidence.  This  sort  of  thing  is  constantly  happening  in 
science.  Investigators  are  constantly  discovering  new 
facts  which  require  the  recasting  of  old  theories.  Now  as 
to  heat,  men  have  had  a  number  of  interesting  theories. 
It  was  once  supposed  to  be  due  to  the  presence  of  a  mysteri- 
ous fluid  known  as  caloric.  And  as  to  fire,  the  ancient 
Greeks  believed  that  it  was  due  to  the  presence  of  another 


INTRODUCTORY  AS  TO  HEAT  125 

mysterious  thing  which  they  called  phlogiston,  a  thing  which 
every  inflammable  substance  was  supposed  to  possess.  In 
the  process  of  burning,  the  phlogiston  was  supposed  to 
escape. 

The  Kinetic  Theory.  —  But  the  modern  explanation  of 
heat  is  based  upon  the  kinetic  theory.  You  learned  in  the 
last  chapter  that  the  theory  of  gravitation  explains  many 
things.  But  you  will  find  that  this  kinetic  theory  is  even 
more  remarkable  as  to  the  number  of  things  that  it  explains. 
You  have  just  learned  that  kinetic  energy  means  energy 
at  work.  Now  the  kinetic  theory  is  a  theory  of  motion. 
You  have  learned  that  all  substances,  whether  gaseous, 
liquid,  or  solid,  are  composed  of  very  small  particles.  A 
single  drop  of  water  is  composed  of  many  millions  of  these 
particles.  They  are  called  molecules,  and  the  kinetic  theory 
is  the  theory  that  molecules  are  in  motion.  That  is  to  say, 
they  move  as  much  as  conditions  permit  them  to  move. 
In  a  solid  substance  this  motion  is,  of  course,  very  limited, 
while  in  a  gas  it  is  not  so  limited.  This  kinetic  theory 
helps  us  understand  why  gases  diffuse  so  rapidly.  It  also 
helps  us  understand  why  liquids  evaporate;  molecules 
at  the  surface  are  continually  flying  off  into  space.  This 
theory  is  too  complex  for  full  explanation  in  this  book, 
but  even  a  very  general  idea  of  it  will  help  you  understand 
many  common  phenomena. 

If  heat  is  due  to  the  activity  of  molecules,  then  the 
process  of  heating  a  substance  is  simply  a  process  of  mak- 
ing its  molecules  move  more  rapidly,  and  the  rise  in  tem- 
perature of  the  heated  substance  is  one  of  the  results  of 
this  acceleration  (increase  in  speed)  of  its  molecules.  So 
we  may  say  that  heat  is  one  of  the  results  of  molecular 


126  ELEMENTARY  SCIENCE 

motion.  The  motion  of  the  molecules  is  a  cause,  of  which 
heat  is  one  of  the  effects. 

If  we  explain  heat  as  a  phenomenon  of  molecular  motion, 
it  is  easy  to  understand  that  it  is  a  form  of  energy.  This  is 
further  evident  when  we  consider  what  results  from  the 
transmission  of  heat  to  the  water  in  the  boilers  of  a  steam- 
engine.  We  know  that  the  heat  in  the  boiler  of  a  locomo- 
tive, by  means  of  the  pressure  of  the  steam  it  produces, 
can  be  changed  into  motion.  We  also  know  that  motion 
can  be  changed  into  heat  again.  We  change  motion  into 
heat  every  time  we  strike  a  match;  even  a  slight  rub 
develops  enough  heat  to  set  fire  to  the  sensitive  chem- 
icals that  compose  the  "head"  of  a  match.  Even  safety- 
matches  require  a  slight  rub  on  the  specially  prepared 
surface,  and  this  rub  increases  the  heat  just  enough  to 
make  the  match  ignite.  This  principle  is  also  evident 
when  a  train  gets  a  "hot  box."  We  know  that  this  heat 
is  a  result  of  friction.  We  know  that  lubricants  (oil,  graph- 
ite, vaseline,  etc.)  must  be  used  in  machines  wherever  there 
is  danger  of  overheating  on  account  of  friction.  Friction 
is  the  rubbing  together  of  two  bodies.  Evidently,  there 
can  be  no  friction  unless  there  is  motion.  It  was  probably 
by  means  of  friction  that  man  first  obtained  fire.  We  do 
not  know  this  to  be  true,  but  it  seems  probable. 

Think  then  of  the  surprise  and  terror  of  that  ancient 
savage  who  first  discovered  fire !  Picture  him  as  he  may 
have  squatted  in  the  sun  on  some  hot  beach.  About  him 
lay  plenty  of  driftwood,  dry  as  tinder,  as  you  yourself 
may  have  seen  it  on  lake  or  ocean  beaches.  He  rubbed  the 
sides  of  two  pieces  briskly  together,  perhaps  to  fashion 
one  into  a  shape  he  wanted,  perhaps  just  to  be  doing  some- 
thing. As  he  rubbed  hard  and  fast,  suddenly  he  noted  a 


INTRODUCTORY  AS  TO  HEAT  127 

little  curl  of  smoke  arising.  This  was  interesting,  but 
not  perhaps  alarming.  Anyhow,  he  kept  on  rubbing  to  see 
what  would  happen.  Suddenly  there  came  a  spark,  and 
then  a  burst  of  flame.  The  dry  wood  caught  and  blazed. 
When  this  happened,  the  discoverer  of  fire  probably 
howled  in  terror  and  ran  for  his  life.  We  can  almost  see 
him  jabbering  in  his  excitement  to  his  family,  telling  of  the 
new  devil  he  had  discovered.  Perhaps  the  next  day  they 
visited  the  place,  and,  since  no  harm  came  of  that,  they 
may  have  repeated  the  experiment.  But  in  the  nature  of 
things  it  must  have  been  a  good  while  before  they  found 
out  how  to  use  this  new  discovery  for  their  own.  comfort 
and  convenience. 

Bonfires  probably  warmed  them  on  cool  days  long  be- 
fore they  discovered  the  art  of  cooking.  However,  it 
is  easy  to  see  that  the  discovery  of  fire  ultimately  made  a 
great  difference  in  the  ancient  world.  For  fire  is  the 
means  by  which  the  wonderful  energy  of  heat  is  placed 
at  man's  service.  Sometimes  it  escapes  his  control  and 
works  with  destructive  violence.  Men  doubtless  saw  fire 
coming  from  volcanoes  before  they  learned  how  to  pro- 
duce it  for  themselves;  doubtless  they  thought  of  it  as 
some  terrible  thing  which,  if  liberated  from  earth's  interior, 
would  destroy  them  all.  Perhaps  the  first  discoverer  of 
fire  thought  he  had  liberated  it.  In  any  case,  it  is  not 
difficult  to  understand  why  there  were  fire-worshippers 
in  those  days. 

QUESTIONS 

1.  What  is  meant  by  "cold"? 

2.  Why  do  we  "strike"  matches? 

3.  How  did  men  obtain  fire  before  matches  were  invented? 

4.  What  is  a  "hot  box"? 


CHAPTER  XVII 
MEASUREMENTS  OF  HEAT 

You  learned  in  the  preceding  chapter  that  thermometers 
and  calorimeters  are  instruments  for  the  measurement  of 
heat.  A  thermometer  measures  temperature,  which  is 
the  degree  or  intensity  of  heat,  while  a  calorimeter  measures 
its  amount.  Both  instruments  are  extremely  useful. 

The  Thermometer.  —  The  most  familiar  form  of  ther- 
mometer is  the  mercury  thermometer,  to  which  you  gave 
some  attention  when  studying  the  barometer  (see  page 
58).  It  consists  of  a  glass  bulb  and  a  tube  of  very  small 
caliber  arising  from  the  bulb  (see  Fig.  55).  Mercury  fills 
the  bulb  and  rises  and  falls  in  the  tube  as  variations  in 
the  intensity  of  heat  cause  it  to  contract  or  expand. 

Since  we  have  noted  that  increase  of  heat  is  associated 
with  increase  of  the  activity  of  molecules,  it  does  not 
surprise  us  to  find  that  this  involves  expansion  of  the  sub- 
stances heated.  But  changes  of  temperature  produce 
other  effects  besides  expansion  and  contraction.  They 
may  change  the  state  of  a  substance,  as  from  solid  to  liquid 
to  gas,  as  in  the  case  of  water.  Or  they  may  produce 
changes  in  its  quality  which  are  not  so  obvious.  Thus, 
in  tempering  steel,  the  quality  given  to  the  steel  depends 
upon  the  temperature  changes  to  which  it  has  been  sub- 
jected. All  such  changes  are  due  to  changes  in  the  nature 
of  the  molecules, 

J28 


MEASUREMENTS  OF  HEAT 


129 


CENT.      PAHH. 


But  to  go  back  to  the  thermometer,  and  the  expansions 
and  contractions  which  it  records.  If  increase  of  heat 
causes  expansion  of  substances  in  general,  then  we  may 
expect  that  the  glass  of  the  thermometer  will  expand 
when  the  mercury  expands,  and  contract  when  it  contracts. 
And  it  does.  Evidently,  if  the  glass  ex- 
panded and  contracted  at  the  same  rate  as  the 
mercury,  the  thermometer  would  be  value- 
less. But  it  does  not.  The  glass  expands  100° 
and  contracts  only  one-seventh  as  much  as  w° 
the  mercury.  Hence,  what  a  thermometer  M° 
actually  records  is  the  di/erence  between  the  70° 
expansion  and  contraction  rates  of  mercury  w° 
and  glass,  and  its  value  as  a  recording  in-  80" 
strument  depends  on  the  fact  that  there  is  *°° 
such  a  great  difference  between  these  rates.  80° 

In  the  manufacture  of  thermometers  mer-  20° 
cury  is  poured  in  at  the  top  of  the  tube  ™* 
(then  open)  until  it  fills  the  bulb  and  part  of  °° 
the  tube.  Then  heat  is  applied  until  the  -10° 
mercury  has  expanded  to  the  top,  and  the  "17-78° 
top  is  closed  by  melting  (fusing)  the  glass 
at  that  point.  Then  the  bulb  is  surrounded 
by  finely  broken  ice.  When  the  mercury 
has  stopped  falling  (contracting)  the  point 
at  which  it  stands  is  marked  on  the  glass. 
That  point  is  called  the  freezing-point — the  freezing-point, 
that  is,  of  pure  water;  different  substances  have  different 
freezing-points.  The  next  thing  is  to  mark  the  boiling- 
point.  This  seems  a  perfectly  simple  thing  to  do  until 
you  remember  that  the  boiling-point  of  water  is  not  al- 
ways the  same.  You  recall  that  water  boils  at  a  much 


FIG.  55. — The  mer' 
cury  thermom~ 
eter. 


130  ELEMENTARY  SCIENCE 

lower  temperature  on  mountains  than  it  does  at  sea- 
level;  that  its  boiling-point  depends  upon  the  atmos- 
pheric pressure.  Evidently,  it  is  desirable  to  mark  ther- 
mometers with  reference  to  the  boiling-point  of  water  at 
a  fixed  and  known  pressure.  So  the  unfinished  thermome- 
ter is  placed  with  its  bulb  and  thread  of  mercury  com- 
pletely enveloped  in  steam  which  rises  from  perfectly 
pure  water,  the  atmospheric  pressure  being  known  to  be 
standard.  The  point  at  which  the  mercury  stops  rising  is 
marked;  this  is  the  boiling-point. 

The  next  thing  to  do  is  to  graduate  the  thermometer; 
that  is,  to  mark  it  off  into  degrees.  The  number  of  degrees 
to  be  marked  off  between  the  freezing-point  and  the  boil- 
ing-point depends  upon  what  kind  of  thermometer  it  is 
to  be.  If  it  is  to  be  a  Centigrade  thermometer,  there  will 
be  just  one  hundred  degrees  between  these  two  points, 
the  freezing-point  being  zero.  (Centigrade  means  "hun- 
dred degrees.")  The  degrees  below  freezing  are  indicated 
by  a  minus-sign. 

But  if  it  is  to  be  a  Fahrenheit  thermometer,  the  freez- 
ing-point will  be  marked  32°  and  the  boiling-point  212°,  the 
interval  between  these  two  points  being  marked  off  into  180 
equal  parts,  each  part  representing  one  degree.  Similar  di- 
visions are  made  both  above  the  boiling-point  and  below  the 
freezing-point.  The  zero  on  the  Fahrenheit  thermometer 
is  the  lowest  temperature  obtainable  by  using  a  mixture 
of  ice  and  salt,  such  as  is  used  in  freezing  ice-cream. 

The  thermometers  in  ordinary  use  in  America  are 
Fahrenheit,  but  in  nearly  all  scientific  work  and  generally 
in  Europe  the  Centigrade  is  used.  The  Fahrenheit  is  used 
by  the  United  States  Weather  Bureau  and  by  physicians. 

Fahrenheit  is  named  from  Gabriel  Fahrenheit,  a  Ger- 


MEASUREMENTS  OF  HEAT  131 

man  scientist,  who  introduced  the  use  of  mercury  in  ther- 
mometers in  1720,  and  established  the  scale  which  bears 
his  name.  The  Centigrade  instrument  was  introduced 
by  Anders  Celsius,  a  Swedish  astronomer,  about  1742; 
it  is  sometimes  called  the  Celsius  thermometer. 
F.  is  used  as  abbreviation  for  Fahrenheit,  and 
C.  for  centigrade. 

At  —  38.8°  C.  mercury  freezes.  Obviously, 
then,  a  mercury  thermometer  cannot  be  used  for 
reading  temperatures  lower  than  this.  Some 
substances  with  a  still  lower  freezing-point  must 
be  substituted.  Alcohol  does  not  freeze  until 
—  in0  C.  is  reached.  So  alcohol  thermometers 
are  used  on  arctic  expeditions  or  whenever  very 
low  temperatures  are  to  be  recorded. 

The  thermometer  which  Galileo  invented  in 
J593  (see  page  124)  was  quite  a  different  instru- 
ment. It  depended  on  air-pressure  to  maintain  the  col- 
umn from  which  the  reading  was  made.  The  bulb  was 
at  the  top  of  the  tube  instead  of  at  the  bottom,  and 
was  large  instead  of  small.  The  tube  was  open  at  the 
lower  end,  and  that  end  was  immersed  in  some  colored 
liquid  (see  Fig.  56).  To  start  this  apparatus  the  air  in  the 
bulb  was  heated.  Then  as  this  air  cooled  down  to  the  tem- 
perature of  its  surroundings,  it  contracted,  and  the  force  of 
atmospheric  pressure  caused  the  ascent  of  the  liquid  in  the 
tube.  (Compare  with  the  action  of  a  suction-pump.)  Evi- 
dently, the  liquid  in  the  tube  would  rise  as  the  air  above 
it  cooled,  and  would  descend  as  this  air  became  warm,  thus 
reversing  the  action  of  a  mercurial  thermometer.  The  tem- 
perature was  read  by  means  of  a  graduated  rule  placed 
alongside  the  tube.  Such  a  thermometer  is  unsatisfactory 


-13.2  ELEMENTARY  SCIENCE 

to  record  large  changes  of  temperature,  but  it  is  very  sensi- 
tive to  small  changes  and  is  frequently  used  to-day  in 
experimental  work. 

The  importance  of  the  thermometer  in  connection  with 
the  study  of  weather  is  obvious.  It  is  also  an  invaluable 
instrument  for  the  physician.  The  normal  temperature 
of  the  human  body  is  98.4°  F.  Variations  from  this 
temperature  indicate  disease.  Especially  in  the  case  of 
fevers,  careful  records  of  the  temperature  of  the  patient 
must  be  kept  in  order  to  guide  the  physician  in  his  treat- 
ment. In  commercial  processes  such  as  the  refinement  of 
oil  or  sugar,  or  the  manufacture  of  steel,  temperature  read- 
ings are  of  the  highest  importance,  since  they  indicate 
physical  changes  which  greatly  affect  the  character  of  the 
product.  Scientific  kitchen  management  also  calls  for 
frequent  use  of  the  thermometer  to  show  the  heat  of  an 
oven  or  for  other  purposes. 

The  Unit  of  Heat.  —  The  unit  of  heat,  measured  as  to 
its  amount,  is  the  calory.  It  is  that  amount  of  heat 
necessary  at  standard  pressure  to  raise  the  temperature 
of  i  cc.  (cubic  centimeter)  of  water  i°  C.  It  is  also  the 
amount  of  heat  which  i  cc.  of  water  gives  off  when  its 
temperature  falls  i°  C.,  which  is  what  we  should  expect 
to  be  the  case,  according  to  the  law  of  conservation  of 
energy.  In  other  words,  the  amount  of  heat  -which  a  sub- 
stance acquires  when  its  temperature  rises  a  certain  amount 
is  identical  with  the  amount  it  gives  of  when  its  temperature 
falls  by  the  same  amount.  This  has  been  proved  many 
times  by  actual  measurements. 

The  number  of  calories  needed  to  raise  the  temperature 
of  a  substance  i°  C.  (or  the  number  which  it  gives  out 


MEASUREMENTS  OF  HEAT  133 

when  its  temperature  falls  i°  C.)  is  called  the  specific 
heat  of  that  substance.  You  have  already  learned  that  some 
substances  "heat  up"  more  rapidly  than  others,  which 
goes  to  show  the  fact  that  substances  vary  as  to  their 
specific  heat.  The  lower  the  specific  heat,  the  more  quickly 
a  substance  heats  up.  Thus  mercury,  whose  specific 
heat  is  low,  heats  up  thirty  times  as  quickly  as  water; 
that  is,  to  raise  i  cc.  of  mercury  i°  C.  requires  the  applica- 
tion of  only  one-thirtieth  of  a  calory. 

With  the  exception  of  hydrogen,  water  has  the  highest 
specific  heat  of  any  known  substance.  Since,  therefore, 
water  absorbs  more  heat  than  other  substances  as  its  tem- 
perature rises,  it  follows  that  it  gives  out  more  heat  as  its 
temperature  falls.  So  we  perceive  why  water  is  so  useful 
in  heaters,  in  hot- water  bags,  or  in  foot- warmers;  no  other 
substance  could  be  used  which  would  give  out  so  much  heat. 
Have  you  ever  noticed  how  much  more  likely  you  are  to 
burn  your  mouth  with  hot  chocolate  than  with  hot  coffee  or 
tea?  What  is  the  explanation  of  this? 

The  calorimeter,  which  measures  the  number  of  calories 
in  substances,  does  this  by  measuring  the  amount  of 
heat  which  they  give  off  to  a  given  amount  of  water. 
There  are  various  types  of  calorimeters.  The  simplest 
is  a  metal  vessel  covered  with  some  non-conductor  (usually 
felt)  to  prevent  the  loss  or  gain  of  heat  from  the  surround- 
ing air.  In  this  vessel  there  is  a  quantity  of  water  of  known 
volume  and  temperature.  The  substance  to  be  tested 
is  put  into  the  water  and  the  change  produced  in  the  tem- 
perature of  the  water  carefully  noted.  From  this  can 
be  estimated  the  number  of  calories  which  have  been  added 
or  subtracted  by  the  substance  introduced. 


134  ELEMENTARY  SCIENCE 

By  the  same  means  we  can  test  substances  of  equal  tem- 
perature to  determine  which  contains  more  actual  heat 
than  the  other.  Thus  if  equal  amounts  of  iron  and  lead 
be  tested  at  equal  temperatures  it  will  be  found  that  the 
iron  gives  off  over  three  times  as  much  heat  as  the  lead; 
that  is,  the  specific  heat  of  iron  is  three  times  as  great  as 
that  of  lead. 

Calorimeter  tests  are  especially  important  in  determining 
relative  values  of  fuel,  especially  of  different  kinds  of  coal. 
They  are  also  important  in  determining  the  "fuel-value" 
to  the  body  of  various  kinds  of  heat-producing  foods.  But 
such  tests  as  these  require  a  more  complicated  apparatus 
than  the  one  described.  They  require  that  the  substance 
to  be  tested  be  actually  burned  under  water.  This  is 
done  in  a  closed  vessel  by  means  of  electricity  and  com- 
pressed oxygen.  The  heat  which  they  impart  to  the 
water  is  then  computed.  Of  course  this  is  very  delicate 
work,  since  it  is  absolutely  necessary  that  all  foreign 
sources  of  heat  gain  or  loss  be  either  eliminated  or  else 
known  so  accurately  that  they  can  be  allowed  for  in  the 
computations. 

QUESTIONS 

1.  Explain  the  principle  of  the  thermometer. 

2.  How  are  thermometers  made? 

3.  What  is  the  Difference  between  Centigrade  and  Fahrenheit  ther- 

mometers ? 

4.  What  kind  of  thermometer  is  used  on  arctic  expeditions,  and 

why? 

5.  Explain  Galileo's  thermometer. 

6.  What  is  the  calory? 

7.  What  is  "specific  heat"? 

8.  Describe  the  calorimetw. 


CHAPTER  XVIII 
HEAT  AND   COLD 

What  Cold  Is.  —  Darkness  is  lack  of  light.  Black,  as 
you  may  have  heard,  is  not  really  a  color.  It  is  lack  of 
color.  Similarly,  cold  is  lack  of  heat.  It  is  not  a  positive 
thing.  It  is  a  negative  thing. 

Yet  we  are  in  the  habit  of  thinking  of  heat  as  one  thing 
and  of  cold  as  another.  We  speak  of  cold  as  a  positive 
cause  of  certain  effects,  when  the  cause,  strictly  speaking, 
is  withdrawal  of  heat.  One  may  say  on  a  cold  day  that 
the  cold  "gets  into  the  house,"  or  "gets  into  one's  bones," 
when  really  it  is  heat  that  gets  out.  Ask  any  one  why  ice- 
cream freezes,  and  he  is  likely  to  say  that  it  is  "because  the 
cold  of  the  ice  gets  into  the  freezer,"  when  really  it  is  be- 
cause the  heat  of  the  cream  gets  out  of  the  freezer. 

It  is  not  important  to  correct  such  inexactness  of  speech, 
but  it  is  important  to  correct  an  inexactness  of  ideas  which 
may  lie  behind  such  speech.  That  is,  there  need  be  no 
objection  to  speaking  of  cold  in  the  way  we  do,  if  we  under- 
stand the  real  facts  involved.  Many  such  inaccurate 
forms  of  speech  have  come  into  such  common  and  general 
use  that  it  is  practically  impossible  to  correct  them.  So, 
in  such  cases,  it  is  not  so  important  to  say  what  we  mean 
as  it  is  to  know  what  we  mean  when  we  say  it;  the  form 
of  speech  may  be  inaccurate,  and  yet  through  usage  and 
general  understanding,  it  may  have  come  to  convey  an 
accurate  meaning.  So  in  this  chapter  we  shall  use  the  word 
cold  when  we  really  mean  lack  or  low  amount  of  heat. 
135 


136  ELEMENTARY  SCIENCE 

Freezing.  —  Let  us  take  first  the  case  of  the  freezing  of 
ice-cream  by  means  of  surrounding  it  with  a  mixture  of 
ice  and  salt.  This  seems  a  simple  process,  yet  when  we 
stop  to  analyze  what  happens  and  why  it  happens,  we  find 
that  it  is  by  no  means  as  simple  as  it  seems.  We  also  find 
that  an  understanding  of  this  process  helps  us  to  under- 
stand many  other  phenomena  of  nature. 

As  we  turn  the  handle  of  the  freezer  we  note  that  the 
mixture  of  salt  and  ice  begins  to  melt.  It  melts  more 
rapidly  than  it  would  if  we  did  not  stir  it  somewhat  by 
our  turning.  Evidently,  the  stirring  stimulates  the  action. 
Similarly,  we  saw  that  stirring  stimulates  the  absorption 
of  air  by  water.  Now  if,  after  some  turning,  we  place  our 
hand  on  the  metal  cylinder  which  contains  the  cream,  we 
find  that  it  is  very  cold,  colder  even  than  ice.  Why  is 
this  and  how  has  it  happened  ? 

What  has  happened  is  that  heat  (a  form  of  energy)  has 
been  withdrawn  from  the  contents  of  the  freezer.  Now 
heat  is  a  result  of  the  motion  of  molecules.  Therefore, 
reduction  of  the  heat  of  a  substance  means  reduction  of 
the  motion  of  its  molecules.  You  have  noted  in  the  case 
of  water  that  increase  of  heat,  if  we  start  with  ice,  involves 
the  change  of  water  from  a  solid  to  a  liquid  and  finally  to 
a  gas;  in  other  words,  change  of  the  rate  of  molecular 
motions  in  a  substance  involves,  if  carried  far  enough, 
changes  in  the  state  of  that  substance.  Now  if  we  reverse 
this  process,  subtracting  heat  instead  of  adding  it,  we 
may  rightly  expect  the  changes  in  the  state  of  a  sub- 
stance (liquid,  solid,  and  gas)  to  occur  in  reverse  order. 
What  has  actually  happened  in  the  case  of  the  cream  in 
the  cylinder  of  the  freezer?  We  started  with  it  as  a  liquid; 
we  note  that  now  it  is  being  gradually  transformed  to  a 


HEAT  AND   COLD  137 

solid.  We  do  not  have  to  look  at  it  to  tell  this;  it  is  indi- 
cated by  the  fact  that  the  handle  of  the  freezer  gets  harder 
and  harder  to  turn.  Evidently,  then,  the  withdrawal  of 
heat,  which  means  the  reduction  of  molecular  motion, 
also  involves  the  transformation  of  this  liquid  into  a  solid. 

Heat  Conduction.  —  But  how  did  this  heat  get  out 
through  the  wall  of  the  metal  cy Under?  You  may  accept 
the  idea  of  heat  being  due  to  molecular  motion,  but  it 
may  be  a  little  difficult  to  see  how  heat,  if  this  be  its  nature, 
is  transmitted  through  such  a  solid  substance  as  the  metal 
of  this  cy Under.  But  did  you  ever  sting  your  hands  by 
striking  a  basebaU  with  a  bat?  Or  suppose  that  your 
body  were  protected  by  tight-fitting  metal  and  some  one 
were  to  strike  you  hard  with  a  ball  or  with  a  bat.  Wouldn't 
you  feel  the  effect  of  the  blow  ?  Then  think  of  the  milUons 
of  milUons  of  blows  which  the  molecules  of  a  Uquid  or  a 
gas  are  constantly  striking  against  the  waUs  of  any  vessel 
which  contains  them.  These  waUs,  like  the  wood  of  a 
ball  bat,  are  capable  of  transmitting  the  effects  of  blows 
struck  upon  them,  and  the  phenomenon  of  the  transmission 
by  soUds  of  molecular  blows  struck  upon  them  is  caUed 
the  conduction  of  heat.  Some  substances  are  better  con- 
ductors of  heat  than  others;  that  depends  upon  the  nature 
of  their  molecular  construction.  Wood  is  a  poor  conductor 
of  heat,  while  metals  are  good  conductors.  You  would 
have  a  great  deal  of  trouble  in  freezing  ice-cream  if  a  wooden 
cyUnder  were  substituted  for  the  metal  one.  Why  do 
teakettles  and  coffee-pots  usually  have  wooden  handles  ? 

But  what  becomes  of  the  heat  that  passes  out  through 
the  metal  cylinder?  Evidently,  according  to  the  law  of 
the  conservation  of  energy,  something  must  happen  to 


138  ELEMENTARY  SCIENCE 

this  heat.  It  cannot  be  lost,  and  certainly  there  is  no 
evidence  that  energy  in  the  form  of  heat  is  passing  into  the 
air  around  the  freezer.  What  actually  happens  to  it  is 
that  it  is  absorbed  by  the  liquefying  salt  and  ice.  The 
heat  energy  which  was  present  in  the  liquid  which  you 
put  into  the  cylinder  is  now  present,  in  another  form,  in  the 
liquid  which  has  formed  outside  the  cylinder. 

You  started  with  a  liquid  inside  and  with  solids  packed 
around  it.  But  you  end  this  process  of  freezing  ice-cream 
with  a  solid  inside  and  with  a  good  deal  of  liquid  outside. 
The  heat  which  was  in  the  liquid  cream  has  now  gone  into 
the  liquid  water  and  salt,  and  as  a  result  you  have  the 
solid  ice-cream,  which  was  what  you  wanted. 

But  what,  you  may  ask,  is  the  need  of  the  salt?  Why 
is  a  mixture  of  salt  and  ice  (a  freezing-mixture)  a  better 
heat  extractor  than  pure  ice  ?  This  is  because  more  heat 
is  absorbed  in  the  melting  of  ice  and  salt  together  than  is 
absorbed  in  the  melting  of  ice  alone;  there  is  the  heat  whiih 
is  absorbed  by  the  melting  ice  plus  the  heat  absorbed  by 
the  melting  (or  dissolving)  salt. 

That  salt  absorbs  heat  when  it  dissolves,  you  can  prove 
by  a  very  simple  experiment.  Immerse  the  bulb  of  a 
thermometer  in  a  tumbler  of  water.  When  you  are  sure 
that  the  mercury  is  stationary,  add  quickly  a  spoonful  of 
salt.  How  much  does  the  mercury  drop  ?  Of  course  the 
mercury  presently  goes  up  again  to  its  original  mark  be- 
cause the  water  absorbs  from  the  air  the  heat  which  it 
gave  up  to  the  dissolving  salt.  But  the  heat  absorbed  by 
the  salt  is  still  there,  there  to  remain  until  the  salt  solidifies 
again  and  gives  it  up. 

The  same  principle  is  involved  in  the  fact  that  the  freez- 
ing-point of  salt  water  is  much  lower  than  the  freezing- 


HEAT  AND   COLD 


139 


point  of  fresh  water.  Ice  forms  in  the  ocean  only  at  much 
lower  temperature  than  in  fresh  water.  Solutions  in  gen- 
eral have  lower  freezing-points  than  their  solvents  taken 
alone.  In  other  words,  the  act  of  dissolving  is  in  itself 
an  absorber  of  heat,  and  in  solutions  this  extra  heat  must 
be  overcome  before  freezing  occurs. 

Here  again  we  have  a  set  of  phenomena  which  are  clear 
and  understandable  only  if  we  keep  the  kinetic  theory 
in  mind.  Let  us  remember  that  molecular  motion  (a 
form  of  energy)  is  much  greater  in  liquids  than  hi  solids. 
Therefore,  a  substance  in  changing  from  a  solid  to  a  liquid 
must  thereby  acquire  more  of  this  form  of  energy,  i,  e.,  of 
heat.  And  when  it  changes  from  a  liquid  to  a  solid,  it 
must  give  up  heat. 

In  the  study  of  geography  you  probably  learned  that  a 
large  lake  has  an  important  effect  in  winter  in  warming 
the  air  which  is  over  it.  This  is  due  to  the  principle  you 
have  just  been  studying;  as  the  water  is  changed  from 
liquid  to  ice  a  great  deal  of  heat  is  given  out,  enough  to 
produce  a  marked  effect  on  the  weather  of  the  surrounding 
region.  Thus,  Chicago  in  winter,  solely  on  account  of 
being  on  Lake  Michigan,  is  often  many  degrees  warmer 
than  other  cities  farther  inland,  and  even  much  farther 
south.  It  is  common  in  very  cold  weather  to  place  tubs 
of  water  near  vegetables  stored  in  cellars.  The  heat  given 
out  by  the  freezing  water  may  be  enough  to  keep  the 
vegetables  from  freezing. 

Also  in  considering  the  phenomena  of  heat  and  cold, 
or,  more  accurately,  of  more  heat  and  less  heat,  let  us 
constantly  remember  that  our  own  feelings  of  heat  and 
cold  are  purely  relative,  and  are  not  at  all  accurate  indica- 
tors of  the  real  state  of  a  substance  as  to  heat.  Heat  is 


140  ELEMENTARY  SCIENCE 

molecular  motion,  but  what  we  commonly  cell  heat  is 
simply  that  degree  of  molecular  motion  which  we  happen 
at  the  time  to  feel,  but  there  are  vast  amounts  of  heat  all 
about  us  which  we  never  feel  at  all.  Heat  is  a  great 
phenomenon  of  nature  of  which  our  senses  alone  enable 
us  to  perceive  only  a  tiny  fraction. 


QUESTIONS 

1.  What  is  cold? 

2.  Explain  conduction  of  heat. 

3.  Explain  how  ice-cream  is  frozen. 

4.  Why  does  the  water  of  the  ocean  not  freeze  at  the  same  tem- 

perature as  the  water  of  the  Great  Lakes  freezes? 

5.  Why  is  Chicago  warmer  in  winter  than  cities  farther  away  from 

the  lake? 


CHAPTER  XIX 
SOME  EFFECTS  OF  HEAT 

You  have  noticed  that  the  principal  effect  of  "change  of 
temperature"  upon  substances  is  to  change  the  volume. 
The  rule  is  that  increase  of  heat  causes  expansion,  and 
decrease  of  heat  causes  contraction.  Change  from  solid 
to  liquid  or  gas  means  expansion,  with  the  exception  of 
the  change  of  ice  into  water.  The  kinetic  theory  helps 
us  very  much  in  explaining  why  changes  in  heat  involve 
changes  in  volume.  You  have  found  that  increase  in  heat 
means  increase  in  the  rate  of  the  movements  of  molecules; 
that  is,  the  hotter  a  substance  is,  the  more  rapidly  its 
molecules  are  moving.  And  the  more  rapidly  its  mole- 
cules move,  the  larger  the  space  they  tend  to  occupy. 
Thus,  the  heating  of  water  "speeds  up"  the  movements 
of  its  molecules;  they  fly  off  at  the  surface  more  abundantly 
(evaporation),  and  thus  we  get.  a  rapid  transformation 
from  liquid  to  gas. 

Can  there  be  such  a  thing  as  the  application  of  heat  to 
a  substance  without  increase  of  its  temperature?  Evi- 
dently, that  depends  upon  what  happens  to  the  heat.  If 
this  heat  simply  increases  the  rapidity  of  the  motions  of 
the  molecules  of  the  substance  to  which  it  is  applied,  then 
the  heat  of  that  substance  increases.  But  if  the  heat 
energy  applied  to  a  substance  is  exerted  in  overcoming 
the  force  of  cohesion  of  its  molecules,  as  in  transforming 
a  solid  to  a  liquid  (or  a  liquid  to  a  gas),  then  the  heat 
energy  applied  to  that  substance  is  transformed,  and 
141 


142  ELEMENTARY  SCIENCE 

does  not  cause  an  increase  in  the  temperature  of  the  sub- 
stance. The  heat  applied  to  it  is  transformed  into  that 
potential  energy  which  a  substance  acquires  when  it  changes 
from  a  solid  to  a  liquid,  and  which  it  gives  up  again  in  the 
kinetic  form  of  heat  when  it  changes  back  to  a  solid.  This 
principle  was  illustrated  in  the  freezing  of  ice-cream. 
The  temperature  of  the  freezing-mixture  does  not  increase 
although  the  temperature  of  the  substance  frozen  decreases 
greatly;  the  heat  withdrawn  from  the  cream  is  used  in 
the  change  of  ice  and  salt  to  liquid;  it  is  "stored"  in  this 
liquid;  it  has  become  potential  rather  than  kinetic.  You 
have  also  learned  that  the  temperature  of  boiling  water 
does  not  increase  no  matter  how  much  heat  is  applied  to 
it,  for  the  heat  is  used  in  the  transformation  of  molecules 
from  the  state  of  liquid  to  the  state  of  gas. 

We  can  demonstrate  this  principle  even  more  simply 
and  clearly  by  placing  in  different  vessels  equal  amounts  of 
ice  and  water,  and  applying  to  them  equal  amounts  of 
heat.  In  the  case  of  the  ice,  the  temperature  of  the  water 
which  results  from  its  melting  will  not  rise  until  all  the 
ice  is  melted;  all  the  heat  applied  is  being  consumed  in 
overcoming  the  solidity  of  the  ice;  it  ceases  to  be  heat, 
having  been  expended  in  the  work  necessary  for  changing 
a  solid  to  a  liquid.  But  in  the  case  of  the  water,  its  tem- 
perature will  begin  to  rise  from  the  moment  that  the  heat 
is  applied.  By  the  time  that  the  ice  in  the  other  vessel 
is  all  melted,  the  water,  to  which  no  more  than  an  equal 
amount  of  heat  is  being  applied,  will  have  increased  in 
temperature  80°  C. 

This  gives  a  simple  means  for  computing  the  amount 
of  heat  consumed  in  the  changing  of  ice  to  water;  the 
amount  of  heat  involved  in  changing  any  solid  to  a  liquid 


SOME  EFFECTS  OF  HEAT  143 

(or  the  reverse)  is  called  the  heat  of  fusion  of  the  substance. 
The  heat  of  fusion  of  ice  is  eighty  calories;  i.  e.,  eighty  cal- 
ories of  heat  are  absorbed  in  the  melting  of  each  gram  of 
ice;  also  for  each  cc.  (gram)  of  water  that  freezes,  eighty 
calories  are  given  out  to  its  surroundings.  Here  again  we 
get  an  idea  of  why  large  bodies  of  water  in  cold  weather 
have  such  a  warming  effect  upon  the  surrounding  regions. 
Since  the  change  of  a  liquid  to  a  solid  involves,  as  you 
have  just  noted,  the  giving  off  of  heat,  what  heat  effect 
would  you  expect  in  connection  with  the  change  of  a 
liquid  to  a  gas;  that  is,  in  connection  with  evaporation? 
The  change  of  liquid  to  solid  is  a  process  of  condensation; 
that  is,  a  process  of  reducing  molecular  motion.  But 
the  change  of  a  liquid  to  a  gas  is  a  process  of  expansion, 
a  process  of  increasing  molecular  motion.  Since  the 
former  process  releases  heat,  it  follows  that  the  latter 
process  absorbs  it.  This  we  find  to  be  the  fact.  Evapora- 
tion is  a  cooling  process;  it  absorbs  heat  from  its  sur- 
roundings. This  you  have  already  learned,  both  in  this 
book  and  elsewhere.  The  added  discomfort  we  feel  on  hot 
days  when  the  atmosphere  is  humid  (full  of  moisture)  is 
due  not  only  to  the  perspiration  which  stays  on  our  skins; 
it  is  due  even  more  to  the  failure  to  have  heat  drawn 
off  from  our  skins  by  the  process  of  evaporation.  Hot 
climates  where  the  air  is  dry  are  even  more  comfort- 
able than  cooler  climates  where  the  humidity  of  the  air 
interferes  with  evaporation.  In  southwestern  United 
States  it  is  common  to  keep  drinking  water  cool  by  placing 
it  in  porous  vessels  and  hanging  these  up  in  the  air;  the 
water  will  become  cooler  than  the  air  which  surrounds  it 
on  account  of  the  heat  carried  off  in  the  process  of  rapid 
evaporation  in  the  dry  air.  Canteens  are  usually  wrapped 


I44  ELEMENTARY  SCIENCE 

in  cloth  for  the  same  purpose  of  keeping  the  water  within 
at  a  temperature  which  makes  it  agreeable  for  drinking. 

Your  attention  has  already  been  called  to  the  exceptional 
character  of  the  expansion  of  water  when  it  freezes,  and 
you  have  considered  the  effects  which  this  exception  pro- 
duces that  are  beneficial  to  man.  Now,  if  we  begin  with 
steam,  we  note  that  reduction  of  the  temperature  of  water 
is  accompanied  by  contraction,  just  as  in  other  substances. 
In  the  change  of  steam  to  liquid  there  is  a  very  con- 
siderable reduction  of  volume,  and  if  you  go  on  lowering 
the  temperature,  this  contraction  continues.  At  what 
point  then  does  this  normal  contraction  cease  and  the  ab- 
normal expansion  of  water  begin?*  We  find  that  this  oc- 
curs at  4°  C. ;  that  is,  at  four  degrees  above  freezing-point 
on  a  centigrade  thermometer,  water  ceases  to  contract  and 
begins  to  expand. 

Now  let  us  consider  what  effects  this  produces  upon  a 
body  of  water  in  cold  weather.  As  the  air  over  the  sur- 
face of  the  water  loses  its  heat,  the  heat  of  the  water 
passes  up  into  it.  As  the  surface-water  loses  heat,  it  in- 
creases in  weight  and  sinks  toward  the  bottom,  while 
the  warmer  and,  necessarily,  lighter  water  beneath  rises 
to  the  surface.  So,  on  account  of  these  movements  of 
the  water  induced  by  the  differences  in  density  which 
accompany  changes  of  heat,  the  whole  body  of  water  tends 
to  become  equally  chilled  throughout.  This  process  con- 
tinues until  the  whole  body  of  water  reaches  a  temper- 
ature of  4°  C.  Then,  evidently,  something  different  is 
sure  to  occur,  inasmuch  as  at  this  point  of  temperature 
the  water  begins  to  expand  instead  of  contract;  it  begins 
to  gain  in  lightness  instead  of  in  weight.  So  the  surface- 
water  which  first  cools  below  4°  C.  does  not  sink;  it  re- 


SOME  EFFECTS   OF  HEAT  145 

mains  at  the  top  and,  if  its  temperature  continues  to  fall 
to  0°  C.,  it  changes  to  ice. 

You  see,  therefore,  that  practically  all  of  the  water 
in  a  lake  or  pond  must  reach  a  temperature  of  4°  C.  before 
any  ice  is  formed.  Consequently,  it  is  easy  to  see  why 
deep  bodies  of  water  take  so  much  longer  to  freeze  over 
than  do  shallow  ones.  Also  it  is  easy  to  see  why  lakes  and 
ponds  do  not  freeze  solid  to  the  bottom,  and  why  fish  and 
other  forms  of  life  get  along  quite  comfortably 
under  the  ice.  The  ice  forms  a  protecting 
barrier  through  which  the  heat  of  the  water 
below  it  penetrates  only  very  slightly.  So 
the  water  under  the  ice  remains  at  about 
4  C.;  this  temperature  permits  life  to  continue 
which  would  cease  to  exist  if  the  freezing- 
point  were  reached. 

You  have  noted  that  changes  in  the  tem- 
perature of  the  water  cause  movements  or  cur-    . 

FIG.  57-— Con- 
rents,  as  a  result  of  which  any  reduction  of  vection  cur- 
temperature  tends  to  become  evenly  distrib- 
uted throughout  the  whole  mass.  (Would  an  increase  of 
temperature  at  the  surface  of  a  body  of  water  have  the 
same  effect?)  Evidently,  here  is  a  means  whereby  heat 
is  distributed  or  transported  from  one  place  to  another. 
You  have  learned  that  heat  migrates  through  solids  by 
means  of  conduction.  You  now  note  that  it  may  migrate 
through  liquids  (and  the  same  is  true  for  gases)  by  means 
of  convection,  for  these  movements  of  liquids  or  gases 
caused  by  their  becoming  warmer  and  therefore  lighter 
than  their  surroundings,  or  colder  and  therefore  heavier, 
are  called  convection  movements  or  convection  currents  (see 
Fig.  57).  It  is  due  to  convection  that  the  warmer  air  is 


146    •  ELEMENTARY  SCIENCE 

at  the  top  of  the  room.  It  is  also  due  to  convection  that 
great  movements  of  air  occur  in  the  earth's  atmosphere 
and  produce  thereby  profound  effects  upon  weather  and 
climate  (see  Fig.  2);  of  this  matter  there  will  be  more 
discussion  in  the  chapter  on  seasons  and  weather. 


QUESTIONS 

1.  Why  is  hotting  water  always  at  the  same  temperature? 

2.  What  is  the  "heat  of  fusion"  of  ice,  and  how  would  you  mea- 

sure it  ? 

3.  Explain  why  evaporation  is  a  cooling  process. 

4.  Why  do  we  feel  more  uncomfortable  on  hot  moist  days  than 

on  hot  dry  days? 

5.  Why  will  drinking  water  keep  cool  if  put  into  a  porous  vessel  ? 

6.  Why  does  it  take  longer  to  freeze  a  deep  body  of  water  than  a 

shallow  one? 

7.  What  are  convection  currents? 


CHAPTER  XX 
THE  ATMOSPHERE 

The  Sun  the  Source  of  the  Earth's  Heat.  —  The  sun  is 

the  great  source  of  heat  for  the  earth.  Its  heat  comes  to 
us  through  the  atmosphere,  and  if  it  were  not  for  the 
effects  which  the  atmosphere  produces  upon  this  heat,  the 
earth  would  be  very  different  from  what  it  is;  indeed,  if 
it  were  not  for  these  atmospheric  effects,  life  as  it  now 
exists  would  perish.  It  could  not  endure  the  great  heat 
of  day  and  the  intense  cold  of  night,  which  we  should 
have  were  it  not  for  the  protecting  and  modifying  effects 
of  the  atmosphere.  Evidently,  then,  to  understand  the 
relations  between  life  and  heat,  and  all  the  other  effects 
which  the  sun  has  upon  the  earth,  it  is  quite  necessary  to 
have  in  mind  a  picture  of  the  atmosphere. 

Already  you  are  familiar  with  a  number  of  important 
facts  about  the  atmosphere.  You  know  that  atmosphere 
and  air  are  words  which  refer  to  the  same  thing.  The 
principal  difference  in  these  words  is  that  atmosphere  is 
commonly  used  to  indicate  all  of  the  air,  while  air  is  com- 
monly used  to  indicate  only  part  of  the  atmosphere.  Thus 
we  say  that  we  breathe  air.  To  say  that  we  "breathe  at- 
mosphere" would  express  the  same  fact,  but  would  express 
it  awkwardly. 

What  the  Atmosphere  Is.  —  You  nave  learned  that  the 
atmosphere  is  a  mixture  of  gases.  You  have  learned  that 
it  has  weight,  exerting  a  pressure  at  sea-level  of  about 

147 


148  ELEMENTARY  SCIENCE 

fifteen  pounds  to  the  square  inch.  You  have  learned  that 
this  pressure  is  used  by  man  in  the  performance  of  cer- 
tain kinds  of  work.  You  have  learned  that  this  atmos- 
pheric pressure  is  less  on  mountain  tops  than  it  is  at  sea- 
level,  and  that  this  lessening  of  the  pressure  is  gradual, 
being  so  definitely  related  to  altitude  that  by  means  of  a 
pressure  recorder  (the  aneroid  barometer)  we  may  esti- 
mate the  altitude  wherever  we  are. 

Density  of  Atmosphere.  —  This  last  fact  about  atmos- 
phere indicates  that  the  higher  one  goes  the  thinner  it  gets. 
What  you  know  about  the  kinetic  theory  (see  page  125) 
makes  you  realize  that  this  thinning  out  of  the  atmos- 
phere as  you  go  up  means  that  the  molecules  of  the 
gases  which  compose  it  get  farther  and  farther  apart; 
the  higher  you  go  the  fewer  the  molecules  to  the  cubic 
centimeter,  until  presently,  as  is  well  known  from  the 
experiences  of  balloonists  and  mountain  climbers,  the  air 
becomes  so  rarefied  that  it  is  difficult  to  breathe.  In 
balloon  ascents  made  for  height,  oxygen  in  metal  con- 
tainers is  taken  along  for  breathing  purposes. 

Balloons  without  passengers  have  gone  up  as  high  as 
eighteen  miles,  showing  that  even  at  that  height  the 
atmosphere  is  sufficiently  dense  to  support  them.  The 
altitude  record  for  balloons  with  passengers  is  between  six 
and  seven  miles.  At  that  height,  air  for  breathing  may  be 
artificially  supplied,  but  great  discomfort  results  from  the 
intense  cold. 

Extent  of  Atmosphere.  —  Other  and  very  important  evi- 
dence of  the  upward  extent  of  the  atmosphere  is  obtained 
from  computing  the  height  of  meteors  (shooting-stars). 


THE  ATMOSPHERE  149 

Meteors  are  solid  bodies  which  come  from  the  heavenly 
spaces  beyond  the  earth's  atmosphere.  They  move  through 
space  with  great  rapidity,  as  does  the  earth  itself.  If  they 
approach  near  enough  to  the  earth  to  be  affected  by  its 
mass  attraction  (gravitation),  they  may  be  drawn  into  our 
atmosphere.  When  they  enter  it  they  are  intensely  cold; 
the  temperature  of  the  space  from  which  they  come  has 
been  estimated  to  be  about  —  459°  F.  But  as  they  plunge 
downward  at  tremendous  speed  they  become  red  hot 
through  friction,  for  now  they  are  travelling  through  real 
substance  instead  of  through  empty  space.  So  they  glow 
and  we  see  them  as  shooting-stars.  Usually  they  are  con- 
sumed by  heat  before  they  reach  the  solid  surface  of 
earth. 

Meteors  have  been  observed  that  were  as  high  up  as 
two  hundred  miles.  So  the  outer  limits  of  the  atmosphere 
must  extend  beyond  even  that  height,  for  meteors  evi- 
dently do  not  begin  to  glow,  and  thereby  become  visible, 
until  they  have  already  passed  far  within  the  outer  limits. 

Now  perhaps  we  can  form  a  mental  picture  of  this 
great  "envelope"  of  earth,  the  atmosphere.  We  see  that 
it  is  really  a  part  of  earth  ;  that  we  live  at  the  bottom  of  a 
great  sea  of  air  which  stretches  for  many  miles  above  us. 
Yet  we  know  that  most  of  the  atmosphere,  computed  by 
its  density,  lies  close  to  earth;  half  of  it,  by  weight,  lies 
under  a  height  of  3.6  miles  above  sea-level,  while  three- 
fourths  of  it  lies  under  6.8  miles.  Above  that  level,  it 
thins  out  gradually  to  nothingness,  and  we  may  conceive 
of  its  outer  limits  as  like  the  fine  spray  from  the  top  of 
a  fountain;  its  particles  bound  off  into  space,  and  are 
then  drawn  back  again  by  force  of  gravity.  Beyond  these 
vague  outer  limits  of  our  atmosphere  are  the  enormous 


j  50  ELEMENTARY  SCIENCE 

empty  spaces  through  which  earth  and  the  other  heavenly 
bodies  move  in  their  orbits. 

Relations  Between  Atmosphere  and  Heat.  —  Now  the 

thing  for  us  to  do  is  to  consider  this  atmosphere,  and  the 
substances  which  are  in  it,  and  then  to  see  what  effect  it 
must  necessarily  produce  upon  the  distribution  of  the  heat 
which  comes  from  the  sun.  Then  we  shall  perceive  that, 
like  soil  and  water  and  light  and  heat  itself,  the  atmosphere, 
through  its  effects  on  heat,  as  well  as  through  its  supply 
to  us  of  the  air  we  breathe,  is  one  of  these  great  facts  of 
nature  which  has  determined  and  continues  to  determine 
the  nature  of  life.  We  shall  see  how  small  we  seem  to 
be  as  compared  with  these  enormous  facts  and  forces  of 
nature  which  permit  us  to  exist.  We  shall  also  see  that 
the  atmosphere  through  the  effects  which  heat  produces 
upon  it  is  the  chief  thing  which  determines  the  distribu- 
tion of  water;  similarly,  through  the  effects  which  it  pro- 
duces upon  heat  it  is  a  very  important  factor  in  the  forma- 
tion of  soil.  Soil  formation  and  water  distribution  are, 
of  course,  two  of  the  things  upon  which  life  depends. 
Thus,  more  and  more,  we  come  to  realize  that  heat  and 
the  atmosphere  are  two  great  co-operating  agencies  whose 
relations  determine  the  nature  of  our  lives.  Surely  it  is  im- 
portant that  we  should  have  some  understanding  of  the 
relations  which  exist  between  them. 

Let  us  remember  also  that  air  is  below  us  as  well  as 
above  us.  It  penetrates  the  soil  and  the  rocks,  it  affects 
fertility,  and  the  substances  which  compose  it  become 
dissolved  in  water  and  unite  chemically  with  rock,  an 
action  similar  to  that  which  occurs  when,  through  respira- 
tion, oxygen  unites  with  substances  in  living  bodies. 


THE  ATMOSPHERE  151 

Composition  of  the  Air.  —  You  have  learned  that  air  is 
made  up,  by  weight,  of  four-fifths  of  nitrogen  and  one- 
fifth  of  oxygen.  These  proportions,  it  should  be  noted, 
refer  to  dry  air;  the  figures  usually  given  are  seventy-eight 
per  cent  nitrogen  and  twenty-one  per  cent  oxygen.  This 
leaves,  you  see,  one  per  cent  which  is  made  up  of  other 
gases.  Of  these  other  gases,  carbon  dioxide  is  the  most 
important  to  life,  but  it  forms,  by  weight,  only  three-ten- 
thousandths  of  the  whole  atmosphere.  Other  gases  also 
exist  in  the  atmosphere  in  small  quantities. 

Water  vapor  is  a  gas  whose  presence  in  air  is  very  im- 
portant to  life,  but,  as  you  have  noted,  the  amount  of  this 
gas  varies  so  much  from  time  to  time  and  from  place  to 
place  that  it  is  usually  regarded  as  something  in  the  air, 
rather  than  as  a  part  of  the  air  itself,  and  it  is  dry  air 
only  which  is  considered  when  we  discuss  its  constitution. 

Air  near  the  earth  also  always  contains  more  or  less  of 
dust,  and  of  organic  matter  in  the  form  of  bacteria  and 
spores,  and  the  presence  of  these  in  air  also  has  im- 
portant effects  on  life.  Air  is  a  medium  by  means  of 
which  both  beneficial  and  harmful  bacteria  are  spread. 
By  means  of  it  the  spores  of  fungi  and  of  other  plants 
are  carried  about.  A  dust-like  kind  of  spore  called 
pollen  is  produced  by  flowers,  and  at  certain  seasons  there 
is  much  of  this  in  the  air.  This  air-transport  of  pollen  is 
of  great  importance  to  us  in  that  by  means  of  it  the  pollen 
of  our  most  important  crop  plants  is  conveyed  from  flower 
to  flower,  a  thing  which  is  important  in  connection  with 
the  production  of  seeds. 

You  have  seen  the  "motes"  of  dust  that  are  visible  when 
you  look  through  a  sunbeam.  You  have  seen  them  danc- 
ing in  it,  and  perhaps  you  have  wondered  if  it  could  be 


152  ELEMENTARY  SCIENCE 

true  that  all  the  rest  of  the  surrounding  air  is  likewise 
filled  with  these  tiny  particles.  (These  particles  are  very 
much  larger  than  molecules,  yet  their  behavior  as  you  see 
them  revealed  in  a  bar  of  light  suggests  the  behavior  of 
the  molecules  of  a  gas  as  you  have  learned  of  it  in  study- 
ing about  the  kinetic  theory.)  The  reason  you  can  see 
the  particles  of  dust  in  a  sunbeam  is  that  they  reflect  the 
light  that  strikes  upon  them;  it  is  the  dust  in  the  air  that 
diffuses  or  scatters  the  light  of  the  sun  so  that  there  is  light 
in  shady  places.  Think  how  important  this  is.  If  it 
were  not  for  the  dust  in  the  air,  all  shady  places  would  be 
in  darkness.  The  colors  of  the  sky,  of  the  sunset,  and  of 
the  sunrise  are  all  produced  by  the  effects  of  the  particles 
of  dust  and  water  in  the  air  upon  the  light  which  strikes 
them.  Also,  when  water  vapor  condenses  into  rain-drops, 
these  dust  particles  act  as  points  about  which  the  water 
first  condenses.  Have  you  ever  noticed  the  dust  specks 
in  a  rain-drop  even  when  it  first  strikes  against  a  clean 
pane  of  glass? 

Relations  of  Air  to  Life.  —  The  importance  of  nitrogen 
to  life  you  have  already  learned.  The  importance  of  oxy- 
gen is  familiar  to  us  in  connection  with  the  process  of 
breathing.  You  know  that  in  the  air  which  we  exhale 
from  our  lungs  the  oxygen  has  been  largely  replaced  by 
carbon  dioxide,  oxygen  being  used  in  our  bodies  in  connec- 
tion with  that  essential  life  process  called  respiration.  It 
is  similarly  used  by  all  plants  *  and  animals  as  weU  as 
by  ourselves. 

This  raises  an  interesting  question  as  to  how  the  supply 
of  oxygen  which  is  constantly  being  used  is  constantly 

*  With  the  exception  of  certain  bacteria. 


THE   ATMOSPHERE  153 

maintained.  For  it  is  constantly  maintained,  and  the 
question  of  oxygen-supply  for  future  generations  never 
perplexes  us  as  does  the  question  of  food-supply.  Here 
again  we  find  one  of  the  links  which  binds  animal  life  to 
plant  life;  here  again  we  realize  that  we  are  partners  in 
life  with  plants,  and  that  our  lives  could  not  continue  if 
this  partnership  were  to  be  dissolved.  For  it  is  the  plants 
which,  more  than  anything  else,  maintain  the  supply 
of  oxygen  in  the  atmosphere. 

You  have  learned  the  process  of  food  manufacture  by 
green  plants  in  sunlight.  The  materials  used  in  the 
first  steps  of  this  process  are  water  and  carbon  dioxide; 
the  water  is  obtained  from  the  soil  and  the  carbon  dioxide 
from  the  air.  (Now  you  see  why  that  small  amount  of 
carbon  dioxide  in  the  air  is  so  important  to  life.)  You 
know  that  in  all  manufacturing  processes  raw  materials 
are  transformed  into  finished  products,  and  that  in  connec- 
tion with  this  process  all  of  the  raw  material  is  usually 
not  consumed;  the  parts  not  consumed  are  wastes  or 
by-products  of  the  process  of  manufacture.  This  thing  is 
also  true  of  the  process  of  food  manufacture  by  plants; 
they  use  carbon  dioxide  (C02)  and  water  (H20)  as  raw 
materials.  They  use  all  of  the  carbon  and  of  the  hydrogen 
in  their  finished  product  (carbohydrate  food),  but  they 
do  not  use  all  of  the  oxygen,  and  this  excess  of  oxygen  is 
returned  to  the  air.  It  is  a  sort  of  waste  or  by-product 
in  food  manufacture,  but  it  serves  the  very  important 
purpose  of  keeping  up  the  supply  of  oxygen  which  is  needed 
for  breathing.  When  you  think  that  nearly  all  the  land 
surface  of  the  earth  is  covered,  at  least  half  the  time, 
with  green  plants,  and  that  this  process  goes  on  in  all  of 
them  while  the  sun  shines  upon  them,  you  can  readily  see 


154  ELEMENTARY  SCIENCE 

that  the  total  volume  of  oxygen  which  they  give  back 
to  the  air  is  very  great. 

But  the  answer  to  this  question  of  oxygen-supply  im- 
mediately raises  another  question,  and  so  it  always  is  in 
the  study  of  nature;  the  answer  to  one  question  usually 
raises  many  others.  If  the  plants  use  carbon  dioxide  in 
maintaining  the  oxygen-supply,  how  is  the  carbon-dioxide 
supply  maintained  ?  Perhaps  you  can  think  of  the  answer 
yourself?  Think  of  all  the  burning  and  breathing  that 
goes  on  in  the  world.  From  both  of  these  processes  carbon 
dioxide  is  given  off. 

QUESTIONS 

1.  What  are  the  limits  of  the  earth's  atmosphere,  and  how  have 

they  been  measured? 

2.  What  is  the  composition  of  air? 

3.  Why  can  you  see  dust  particles  in  a  sunbeam? 

4.  Why  is  there  any  light  in  shady  places? 

5.  What  relation  has  dust  to  rain? 

6.  What    maintains    the    supply  of  oxygen  in  the  atmosphere? 

How? 

7.  What  maintains  the  supply  of  carbon  dioxide?    How? 


CHAPTER  XXI 

AIR  AND  WATER  COMPARED.    ATOMS  AND 
ELEMENTS 

You  have  learned  that  water,  like  all  other  substances, 
is  composed  of  molecules.  You  have  noted  that  a  molecule 
of  water  is  composed  of  hydrogen  and  oxygen,  the  chemical 
symbol  indicating  this  composition  being  H2O. 

Now  as  to  air.  You  may  ask,  is  there  such  a  thing  as 
an  air  molecule,  and  if  so,  what  is  its  composition  ?  Already 
you  know  that  air  contains  oxygen,  which  we  use  in  breath- 
ing, and  nitrogen,  and  a  little  carbon  dioxide.  So  we  ask, 
are  these  substances  parts  of  air  molecules,  as  hydrogen 
and  oxygen  are  parts  of  water  molecules,  or  do  they  exist 
in  some  other  relationship  ? 

The  fact  is  that  there  is  no  such  thing  as  an  air  molecule. 
Pure  water  is  composed  of  just  one  kind  of  molecule,  but 
pure  air  is  composed  of  a  number  of  different  kinds.  Water 
is  what  is  called  a  compound,  while  air  is  a  mixture. 

Here  we  have  come  upon  a  very  important  fact.  If  we 
stop  to  consider  it  carefully,  we  shall  get  some  important 
information  as  to  how  all  substances  are  composed.  Water 
a  compound,  and  air  a  mixture  !  The  thing  for  us  to  do  is 
to  see  just  what  is  meant  by  this  word  compound,  and 
then  to  see  why  it  does  not  apply  to  air  as  well  as  to  water. 

Molecules.  —  To  do  this  we  must  look  a  little  further 
into  the  nature  of  molecules.    A  molecule  is  the  least 
155 


156  ELEMENTARY  SCIENCE 

particle  into  which  any  substance  may  be  divided,  and  yet 
remain  the  same  substance.  You  know  that  in  air,  oxygen, 
nitrogen,  and  carbon  dioxide  occur.  Now  the  new  fact 
to  note  is  that  air  is  composed  of  separate  molecules  of 
oxygen,  of  nitrogen,  and  of  carbon  dioxide  (CO2),  which 
are  equally  distributed  in  accordance  with  the  law  of 
diffusion  that  you  have  already  considered.  In  other 
words,  if  all  molecules  were  so  big  that  we  could  see 
them,  and  tell  them  apart  one  from  another,  we  should 
see  that  water  is  composed  of  molecules  which  are  all 
alike.  But  when  we  examine  air,  we  find  it  to  be  chiefly 
composed  of  two  distinct  kinds  of  molecules,  oxygen 
molecules  and  nitrogen  molecules,  and  these  would  be 
symmetrically  arranged  so  that  you  would  find  one  oxygen 
molecule  associated  with  every  four  nitrogen  molecules. 
Then,  about  once  in  every  twenty- five  hundred  molecules 
(not  counting  those  of  water  vapor)  you  would  find  one 
of  carbon  dioxide. 

Thus  we  see  that  a  compound  is  a  substance  all  of  whose 
molecules  are  of  the  same  kind,  while  a  mixture  is  a  substance 
whose  molecules  are  of  more  than  one  kind. 

Elements.  —  It  is  the  work  of  chemists  to  analyze 
substances,  and  thereby  to  discover  of  what  they  are 
composed.  In  this  process  molecules  are  decomposed 
in  order  that  their  composition  may  be  discovered.  Now 
one  of  the  most  important  and  significant  facts  that  all 
this  work  of  the  chemists  has  revealed  is  that  there  are 
only  about  eighty  substances  in  the  world  which  may  not 
be  reduced  into  simpler  substances.  They  have  analyzed 
thousands  upon  thousands  of  different  substances,  decom- 
posing them  as  far  as  possible,  and  the  result  of  all  this 


AIR  AND  WATER  COMPARED  157 

work  indicates  that  all  substances  whatsoever  are  composed 
of  one  or  more  of  these  eighty  simplest  substances.  Just 
as  all  words  are  made  up  of  various  combinations  of  a 
few  letters,  so  all  substances  are  made  up  of  various  com- 
binations of  a  few  elements.  So  you  can  see  that  the 
translation  of  this  word,  analysis,  "loose  back,"  gives  a 
good  idea  of  the  real  nature  of  the  process.  It  is  a  process 
of  "loosing"  molecules  "back"  to  the  elements  of  which 
they  are  composed.  The  opposite  of  analysis  is  synthesis, 
which  means  the  "putting  together"  (by  chemical  combina- 
tion) of  simpler  substances  into  more  complex  ones.  Thus 
you  may  have  heard  of  synthetic  rubber,  which  means  rubber 
manufactured  by  chemical  processes  out  of  simpler  sub- 
stances so  that  the  product  is  practically  equivalent  to 
the  natural  rubber  obtained  from  plants. 

Note  that  new  word,  elements.  It  is  important.  All 
substances,  then,  are  elements  or  compounds  of  elements. 
What  are  these  eighty  elements  or  simplest  substances? 
With  some  of  them  you  are  already  familiar.  Hydrogen, 
oxygen,  and  nitrogen  are  elements.  Others  with  which 
you  are  familiar  are  carbon,  copper,  gold,  iron,  lead,  mercury, 
phosphorus,  platinum,  silver,  sulphur,  tin,  and  zinc.  Evi- 
dently, then,  elements  may  exist  as  liquids,  solids,  or  gases. 
Which  is  the  element  named  above  which  exists  in  liquid 
form? 

Atoms.  —  Now  to  go  back  again  to  our  water  molecule. 
We  have  been  told  that  it  is  composed  of  two  parts  of  hydro- 
gen and  one  of  oxygen.  These  "parts"  of  which  mole- 
cules are  composed  are  called  atoms.  Thus  the  molecule 
of  carbon  dioxide  is  composed  of  one  atom  of  carbon  and 
two  of  oxygen. 


ELEMENTARY  SCIENCE 


The  fact  that  molecules  of  water  contain  two  atoms  of 
hydrogen  to  one  of  oxygen  is  indicated  by  the  fact  that 
when  they  are  decomposed  into  these  two  elements  (which, 
when  liberated,  take  the  form  of  gases)  the  volume  of  hydro- 
gen obtained  is  just  twice  as  great  as  the  volume  of  oxygen. 
Water  may  be  so  decomposed  by  passing  through  it  an 
electric  current;  the  process  is 
called  electrolysis.  Study  Fig.  58. 


FIG.  58.  — Diagram  illustrat- 
ing how  water  is  decomposed 
by  electrolysis;  the  hydro- 
gen collects  in  top  of  tube  H 
and  the  oxygen  collects  in 
top  of  tube  O—  After  Mn.- 
LIKAN  and  GALE. 


Chemical  and  Physical  Changes. 
—  Any  change  in  the  nature  of  the 
molecule  of  a  substance  is  a  chem- 
ical change.  All  other  kinds  of 
changes  are  physical  changes.  The 
change  of  water  to  ice  or  steam  is  a 
physical  change.  The  change  of 
water  to  hydrogen  and  oxygen  is  a 
chemical  change.  Chemistry  is  the  study  of  chemical 
changes.  Physics  is  the  study  of  physical  changes.  Both 
physical  and  chemical  changes  are  constantly  going  on  in 
living  as  well  as  in  non-living  bodies.  Life  itself  is  a  con- 
stant procession  of  changes.  Evidently,  to  understand  liv- 
ing things,  it  is  quite  necessary  first  to  understand  some- 
thing of  physics  and  chemistry. 

You  have  just  noted  that  molecules  are  composed 
of  atoms;  thus  the  water  molecule  is  composed  of  two  atoms 
of  hydrogen  and  one  of  oxygen.  Molecules  composed  of 
few  atoms  (few  both  in  number  and  kind],  like  the  water 
molecule  or  the  carbon-dioxide  molecule,  are  said  to  be 
simple.  But  molecules  composed  of  many  atoms  are  said 
to  be  complex.  The  molecules  of  some  substances  are  very 
complex,  containing  even  hundreds  of  atoms. 


AIR  AND   WATER   COMPARED  159 

Now  it  is  evident  that,  by  chemical  change,  complex 
molecules  may  be  reduced  to  simpler  ones,  or  simple  mole- 
cules may  be  built  up  into  more  complex  ones.  Already 
you  have  considered  a  number  of  cases  of  chemical  changes. 
It  will  be  well  to  reconsider  these,  and  to  determine  in 
each  case  whether  reduction  or  the  building  up  of  mole- 
cules occurs. 

Perhaps  the  most  familiar  of  chemical  changes  is  the 
process  of  burning.  You  have  learned  that,  in  the  burn- 
ing of  wood  or  coal,  gases  are  produced.  Two  of  these 
gases  are  carbon  dioxide  and  water  vapor.  They  result 
from  the  union  of  oxygen  of  the  air  with  carbon  and  hydro- 
gen, which,  before  the  burning  occurred,  were  in  the 
molecules  of  the  fuel.  These  fuel  molecules  are  much 
more  complex  than  the  simple  ones  of  water  and  carbon 
dioxide.  Burning,  therefore,  is  a  case  of  chemical  re- 
duction. We  shall  consider  it  further  in  the  chapter  on 
combustion. 

In  Chapter  V  you  learned  that  in  washing  your  hands 
with  soap  in  hard  water,  the  soap  " unites"  with  the 
mineral  matter  in  the  water,  and  that  thereby  a  dirty 
sediment  is  produced.  The  word  unites,  used  in  this  way, 
indicates  that  a  chemical  change  occurs;  that  molecules, 
or  parts  of  molecules,  of  different  substances  have  united 
in  the  form  of  another  kind  of  molecule.  In  this  case  the 
molecule  of  the  sediment  is  more  complex  than  the  mole- 
cules from  which  it  was  derived,  so  this  change  is  not  a 
reduction  but  a  building  up.  When  a  chemical  change  (or 
reaction)  occurs  which  causes  the  production  of  an  insolu- 
ble substance  in  a  liquid  (as  in  this  case  of  the  reaction 
between  hard  water  and  the  dissolved  soap),  the  insoluble 
substance  is  called  a  precipitate.  So  we  may  say  that  the 


160  ELEMENTARY  SCIENCE 

use  of  soap  in  hard  water  causes  a  sediment  to  be  pre- 
cipitated, or  "thrown  down." 

In  Chapter  XII,  in  studying  the  origin  of  soil,  you 
learned  that  oxygen  and  water  act  on  rock,  producing 
changes  known  as  oxidation  and  hydration.  Rusting  you 
noted  to  be  a  process  which  includes  both  oxidation  and 
hydration;  that  is,  the  chemical  union  of  oxygen  and  water 
with  other  substances.  Here  we  have  another  case  of 
reduction,  similar  to  that  noted  in  burning. 

Then,  in  Chapter  XIII,  in  considering  the  nitrogen-fixing 
bacteria,  you  learned  that  certain  soil  bacteria  are  able  to 
act  upon  the  nitrogen  of  the  air,  and  to  "fix"  it  in  such  form 
that  it  becomes  available  for  the  use  of  plants.  This 
nitrogen-fixing  process  results  in  the  production  of  sub- 
stances called  nitrates,  whose  molecules  are  much  more 
complex  than  the  molecules  of  atmospheric  nitrogen;  it 
is  a  building-up  process. 

Similarly,  from  the  bacteria  of  the  soil  up  to  the  highest 
living  creatures,  all  life  is  based  on  a  constant  succession 
of  chemical  changes  —  actions  and  reactions,  as  they  are 
called.  By  the  process  of  digestion,  the  molecules  of  the 
food  we  eat  are  reduced  to  simpler  substances  which 
enter  our  bodies,  and  there  through  a  long  series  of  changes, 
these  substances  are  built  up  into  more  and  more  complex 
substances,  the  most  complex  of  all  being  protoplasm, 
which  is  the  living  substance  itself. 

So  we  must  have  food  to  build  up  and  maintain  our 
bodies.  But  we  must  also  have  air,  whose  oxygen  is  carried 
by  our  blood  to  all  parts  of  the  body.  There  it  acts  chem- 
ically upon  the  substances  which  have  been  built  up  from 
our  food.  This  action  (oxidation)  is  a  reducing  process, 
like  the  action  you  have  noted  in  the  process  of  burning. 


AIR  AND  WATER  COMPARED  161 

By  means  of  it  (as  in  burning)  energy  is  released.  This 
energy  may  take  the  form  of  heat  or  of  motion.  It  gives 
us  what  we  call  the  manifestations  of  life.  The  substances 
of  our  bodies,  that  are  thus  reduced  through  oxidation  and 
their  energy  released,  become  of  no  further  use  to  us; 
they  are  eliminated  as  excretions.  Thus  we  see  how  very 
fundamentally  chemical  changes  are  a  part  of  the  processes 
of  life. 

QUESTIONS 

1.  What  is  the  great  chemical  difference  between  air  and  water? 

2.  What  is  a  molecule? 

3.  What  is  meant  by  "analysis"  and  "synthesis"? 

4.  What  is  an  atom,  and  what  relation  does  it  hold  to  a  molecule  ? 

5.  How  is  water  decomposed? 

6.  Explain  the  difference  between  a  physical  change  and  a  chem- 

ical change,  giving  examples  of  each. 

7.  Explain  the  chemical  changes  involved  in  the  burning  of  wood. 

8.  Explain  the  rusting  of  iron. 


CHAPTER  XXII 
HEAT  AND   THE  ATMOSPHERE 

Now  we  have  a  picture  in  mind  of  the  atmosphere,  and 
we  know  something  of  the  nature  of  heat.  We  know  that 
it  is  a  form  of  energy  that  tends  to  increase  molecular 
motions  (see  Chapter  XVI),  and  that  what  we  perceive 
as  heat  or  cold  is  simply  the  effect  on  us  of  different  rates 
of  these  molecular  motions.  So  now  our  problem  is  to 
consider  the  heat  of  the  sun  as  applied  to  our  atmosphere, 
to  see  what  effects  this  atmosphere  produces  upon  the 
distribution  of  this  heat,  and  to  realize  what  relations  the 
resulting  phenomena  have  to  life. 

You  have  learned  that  heat  may  be  absorbed.  So  it 
is  easy  for  you  to  realize  that  the  amount  of  solar  (sun) 
heat  that  reaches  the  solid  and  liquid  surfaces  of  earth 
is  much  less  than  the  original  amount  that  comes  to  earth 
from  the  sun.  For  the  heat  "rays"  coming  to  earth 
first  encounter  its  gaseous  surface;  i.  e.,  the  outer  limits 
of  the  atmosphere.  In  passing  through  the  atmosphere 
much  of  the  heat  is  absorbed,  so  that  the  amount  of  it 
which  finally  reaches  the  land  and  water  surfaces  (at  the 
bottom  of  the  great  enveloping  sea  of  air)  is  far  less  than 
that  originally  received  at  the  atmospheric  surface. 

In  view  of  what  has  just  been  said,  why  is  it  that,  gen- 
erally speaking,  the  higher  up  we  go  the  colder  it  gets? 
You  know  that  mountain  tops,  although  nearer  the  source 
of  heat,  are  generally  colder  than  the  land  which  lies  be- 
162 


HEAT  AND  THE  ATMOSPHERE  163 

low  them.  How  can  we  explain  this?  This,  like  most 
other  phenomena  of  nature,  cannot  be  fully  explained 
by  just  one  thing.  A  number  of  things  must  be  taken  into 
consideration.  We  witness  the  effect,  but  to  explain  it 
fully,  we  must  find  a  number  of  causes  or  "factors"  in 
the  problem.  One  of  the  causes  of  the  coldness  of  moun- 
tain tops  is  the  "thinness"  of  the  surrounding  air;  its 
capacity  to  absorb  and  retain  heat  is  much  less  than  that 
of  the  denser  air  which  lies  below.  Other  "contributing" 
causes  will  occur  to  you  as  we  study  our  larger  problem. 
Similarly,  the  polar  regions,  even  at  sea-level,  do  not  get 
warm,  although  at  certain  seasons  they  received  great 
amounts  of  heat.  Here  the  principal  cause  of  coldness  is 
evidently  different  from  that  in  the  case  of  mountain  tops; 
it  is  that  the  heat  received  is  so  largely  absorbed  in  the 
process  of  melting  ice  and  warming  very  cold  water,  that 
the  effects  upon  the  temperature  of  the  atmosphere  are 
relatively  slight. 

Already  you  have  learned  that  it  is  absolutely  contrary 
to  the  nature  of  heat  to  stay  in  the  same  place.  You 
learned  this  when  you  learned  that  heat  is  motion.  Obvi- 
ously, we  cannot  conceive  of  motion  being  at  rest.  To 
be  heat,  it  must  continue  to  be  motion;  the  motion  of 
molecules. 

But,  you  may  say,  what  of  a  thermos  bottle  or  a  fire- 
less  cooker?  Are  not  these  devices  by  means  of  which  we 
confine  heat  and  keep  it  in  one  place?  Stop  to  consider. 
A  thermos  bottle  consists  of  an  inner  receptacle  surrounded 
(except  for  the  opening)  by  a  hollow  metallic  case.  From 
the  hollowness  of  this  metallic  case,  before  it  was  sealed, 
the  air  was  withdrawn,  thus  producing  a  vacuum  (see  Fig. 
59).  Now  a  vacuum  is  a  "non-conductor"  of  heat. 


i64 


ELEMENTARY  SCIENCE 


Hence,  liquids  placed  in  a  thermos  bottle  change  their 
temperature  only  very  slowly.  If  hot,  their  heat  has  no 
easy  path  of  escape  save  through  the  cork,  which  is  itself 
a  poor  conductor.  If  cold,  the  heat  outside  has  similar 
difficulty  in  entrance,  and  so  the  contained  liquid  stays  cold. 
Similarly,  the  fireless  cooker  depends  upon  the  very  slow 
conduction  of  heat  by  the  materials  which  surround  the 
central  receptacle.  Hence,  well-heated  food  placed  within 
it  continues  to  "cook,"  even  in  the  absence  of 
fire.  In  neither  case,  however,  is  heat  actually 
compelled  "to  stay  in  the  same  place."  The 
rate  of  its  migration  is,  of  course,  greatly  re- 
duced, but  the  motion  continues,  even  though 
largely  confined.  In  fact,  the  "whole  purpose 
of  a  thermos  bottle  is  not  to  stop  the  motion 
of  heat,  but  to  preserve  it,  and  it  does  this  by 
confining  it  as  much  as  possible  to  a  limited 
space,  thus  preventing  it  from  dissipating  its 
energy." 

*  The  Sthe7        Evidently,  however,  there  can  be  no  such 
TOtke0tthe     confinement  of  the  sun's  heat  in  the  air  or  in 
vacuum     £ne  soii  or  m  the  water  or  in  whatever  it  is 
that  absorbs  it.     After  it  has  been  first  ab- 
sorbed by  air,  or  rock,  or  water,  its  journey  here  on  earth 
has  but  just  begun.    To  be,  it  must  keep  moving,  and  so 
it  does  keep  moving  about  the  earth,  from  one  substance 
to  another  and  back  again,  and  in  its  movements  it  has 
profound  effects  upon  all  the  life  that  inhabits  the  earth. 
It  can  be  transformed,  and  it  can  pass  by  radiation  out 
into  space  again,  but  it  cannot  be  lost  or  destroyed  or  used 
up.     It  is  energy,  and  you  have  learned  that  energy  is 
indestructible. 


HEAT  AND  THE  ATMOSPHERE  165 

Transfer  of  Heat  at  the  Earth's  Surface.  —  Since 
heat  is  motion,  it  is  better  to  speak  of  the  transfer  of 
it  than  of  the  motion  of  it.  There  are  three  ways  in  which 
heat  is  transferred,  and  all  of  these  are  important  in  con- 
nection with  the  natural  distribution  of  it  over  the  earth's 
surface.  These  three  methods  of  heat  transfer  are  radia- 
tion, conduction,  and  convection.  Conduction  and  con- 
vection you  have  already  studied  (see  Chapters  XVIII 
and  XIX) .  Radiation  we  will  now  consider. 

Radiation.  —  It  is  natural  to  place  radiation  first  in  this 
list,  because  by  this  method  heat  travels  from  the  sun  to 
the  earth;  our  original  supply  of  it  comes  to  us  in  this 
way.  Radiation  means  movement  outward  equally  in  every 
direction;  it  may  be  represented  by  the  radii  of  a  circle, 
or,  better,  of  a  sphere.  When  we  speak  of  the  sun's  "rays" 
we  are  expressing  the  idea  of  radiation.  A  radiator  radi- 
ates heat  out  hi  every  direction  in  a  room. 

Heat  is  not  the  only  form  of  energy  which  travels  by 
radiation.  Light  also  radiates.  Heat  and  light  are  then 
forms  of  radiant  energy.  (What  is  an  example  of  energy 
which  is  not  radiant  ?)  Radiant  energy  travels  in  the  form 
of  invisible  "waves,"  and  it  is  a  kind  of  energy  transfer 
which  does  not  require  substance  through  which  to  move,  as 
is  proved  by  the  fact  that  light  and  heat  come  to  us  through 
enormous  spaces  that  are  practically  empty  of  molecules. 

Now  the  importance  of  radiation  in  connection  with  the 
distribution  of  heat  over  the  earth's  surface  is  due  to  the 
fact  that  the  sun  shines  on  only  half  of  the  earth  at  a  time. 
This  is  a  very  familiar  fact,  but  we  do  not  often  stop  to 
think  what  all  the  consequences  of  it  are.  One  of  the 
most  important  of  them  is  that  while  half  of  the  earth 


1 66  ELEMENTARY  SCIENCE 

is  absorbing  heat,  the  other  half  is  giving  it  off.  While 
the  sunny  side  of  earth  absorbs  heat,  the  shaded  side 
radiates  it.  The  shadow  of  earth  we  call  night,  and  the 
nights  would  all  be  intensely  cold  were  it  not  for  the  giv- 
ing off  from  land  and  water  surfaces  of  heat  which  they 
absorb  by  day. 

The  capacity  of  air  to  absorb  heat  is  obviously  far  less 
than  that  of  land  or  water.  What  capacity  it  has  depends 
largely  upon  the  amount  of  moisture  in  it.  Thus  we  see 
why  in  our  dry  southwest,  or  in  the  Desert  of  Sahara, 
there  is  much  greater  variation  in  temperature  between 
day  and  night  than  there  is  in  regions  of  greater  humidity. 

Now  if  day  be  longer  than  night,  as  in  spring  or  summer, 
evidently  the  amount  of  heat  absorption  will  be  greater 
than  the  amount  of  heat  radiation.  The  land,  the  water, 
and  the  "weather"  all  become  warmer.  But  if  night 
be  longer  than  day,  as  in  fall  and  whiter,  more  heat  is  lost 
than  is  gained,  and  everything  gets  colder.  So  we  see  that 
the  principal  cause  of  the  seasons  is  the  variation  in  the 
lengths  of  day  and  night.  But,  you  ask,  what  causes  these 
variations  of  length?  The  answer  to  that  we  must  post- 
pone till  the  chapter  on  seasons. 

Conduction.  —  This  method  of  heat  transfer  you  con- 
sidered in  connection  with  the  freezing  of  ice-cream.  Con- 
duction requires  the  actual  contact  of  substances  of  differ- 
ent temperature.  It  is  by  conduction  that  our  hand  is 
cooled  when  we  touch  ice,  or  warmed  when  we  touch  a 
stove  or  a  radiator.  In  nature,  conduction  operates  to 
transfer  heat  into  air  which  is  in  contact  with  warmer 
land  or  water  surfaces,  and  thus  it  is  an  important  factor 
in  the  general  distribution  of  heat  which  we  are  considering. 


HEAT  AND   THE  ATMOSPHERE  167 

You  have  already  learned  that  land  and  water  are  better 
heat  absorbers  than  is  air.  Therefore,  it  may  not  surprise 
you  to  learn  that  of  the  total  heat  of  the  air  more  is  acquired 
by  conduction  and  radiation  back  from  land  and  water 
than  is  acquired  by  absorption  directly  from  the  heat- 
rays  of  the  sun.  The  air  acquires  heat  from  the  sun 
only  when  the  sun  shines  on  it,  but  it  is  always  in  a  position 
to  acquire  it  from  land  and  water. 

Convection.  —  You  learned  of  this  process  in  studying 
what  happens  when  a  body  of  water  freezes,  and  you  noted 
then  the  convection  currents  of  the  atmosphere.  They 
are  caused  by  heat,  and  they  in  turn  are  the  causes 
(though  not  the  only  ones)  of  winds  and  of  the  distribu- 
tion of  rain.  Evidently,  then,  they  are  important  factors 
in  the  environment  of  life. 

It  should  not  be  at  all  difficult  for  you  now  to  form  a 
mental  picture  of  the  great  convection  currents  in  earth's 
atmosphere.  You  know  that  the  surface  of  earth  is  hottest 
in  the  equatorial  regions.  Hence,  the  air  over  these  regions 
will  become  warmer  than  the  air  of  the  regions  to  the 
north  and  south.  So  it  rises,  and,  as  it  rises,  cooler  air 
from  north  and  south  comes  creeping  in  to  replace  it. 
As  the  warm  ascending  air  reaches  higher  altitudes,  it 
cools,  and  then  flows  away  toward  the  poles.  Thus  we 
have  a  constant  great  convectional  movement  or  circula- 
tion of  earth's  atmosphere  (see  Fig.  2).  This  alone  is 
simple  enough,  but  we  must  remember  that  the  whole 
matter  of  atmospheric  movements  is  complicated  by  the 
rotation  of  the  earth,  and  by  other  factors.  These  are 
matters  to  be  taken  up  in  the  next  chapters. 


j  68  ELEMENTARY  SCIENCE 


QUESTIONS 

1.  Why  is  it  colder  on  mountain  tops  than  on  plains? 

2.  Why  is  it  colder  at  the  north  pole  than  at  the  equator? 

3.  Explain  the  thermos  bottle. 

4.  In  what  ways  is  heat  transferred? 

5.  How  does  heat  travel  from  the  sun  to  the  earth? 

6.  What  keeps  us  warm  on  summer  nights? 

7.  Why  is  there  much  greater  variation  in  temperature  between 

day  and  night  in  the  Desert  of  Sahara  than  there  is  in  most 
other  places? 

8.  What  is  the  cause  of  the  seasons? 

9.  How  does  the  air  get  most  of  its  heat? 


CHAPTER  XXIII 

THE  SEASONS  AND  THE  SOLAR  SYSTEM 

You  have  learned  that  a  principal  cause  of  what  we  call 
the  seasons  is  the  variation,  at  different  times  of  the 
year,  of  the  lengths  of  day  and  night.  The  longer  the 
day,  the  greater  the  heat,  and  vice  versa. 

From  this  it  appears  to  follow  that  the  longest  day  of 
the  year  should  be  the  hottest  day.  Similarly,  we  noted 
that  it  appears  as  though  those  points  on  the  earth's  sur- 
face nearest  the  sun  (mountain  tops)  would  be  the  hottest. 
But  we  found  that  other  factors  than  nearness  to  the  sun 
affect  this  matter,  and  that,  as  a  matter  of  fact,  moun- 
tain tops  are  cold.  So  as  to  the  longest  days.  Other 
factors  than  length  of  sunshine  affect  the  matter,  and 
they  are  not  the  hottest  days. 

June  21,  as  you  probably  know,  is,  in  the  northern 
hemisphere,  the  longest  day  of  the  year,  and  yet  most  of 
the  heat  of  summer  comes  after  that  date.  Similarly, 
December  22  is  the  shortest  day  and,  similarly,  most  of 
the  cold  of  winter  comes  after  that  date.  Evidently,  the 
time  of  the  greatest  heat  comes  after  the  time  of  the  great- 
est heating  and,  similarly,  the  time  of  least  heat  comes 
after  the  time  of  least  heating. 

You  learned  that,  for  certain  reasons,  the  polar  regions 
never  become  warm,  although  there  may  be  a  great  deal  of 
sunshine  there  at  certain  seasons;  in  fact,  these  regions 
have  the  longest  days  of  all,  since  there,  for  months  at  a 
time,  the  sun  never  sets.  For  similar  reasons,  the  region  in 
169 


170 


ELEMENTARY  SCIENCE 


which  we  live  does  not  "warm  up  "  to  its  warmest  until  after 
the  longest  day,  or  "cool  off"  to  its  coldest  until  after  the 
shortest  day.  What  are  some  of  these  reasons?  If  you  re- 
member what  has  been  said  of  the  relations  of  heat-  to  soil 
and  rock  and  water,  you  can  solve  this  problem  for  yourself. 

Solar  System.  —  It  is  easy  to  see  why  the  seasons  re- 
sult from  changes  in  the  length  of  day,  but  you  cannot 

understand  why  the 
days  change  in  length 
unless  you  under- 
stand the  solar  system. 
The  solar  system  is 
composed  of  the  sun 
and  all  the  bodies 
which  revolve  about 
it,  chief  of  which  are 
the  eight  planets.  Of 
these,  the  earth  is  one. 
All  of  the  planets,  ex- 
cept the  earth,  have 
names  that  were  given 
them  by  the  early  astronomers  in  honor  of  ancient  deities. 
In  the  order  of  their  nearness  to  the  sun,  the  planets  are: 
Mercury,  Venus,  Earth,  Mars,  Jupiter,  Saturn,  Uranus, 
Neptune  (see  Fig.  60). 

The  planets  revoke  about  the  sun,  following  their  orbits, 
and  also  rotate  on  their  axes.  In  the  case  of  earth,  one 
complete  rotation  takes  twenty-four  hours,  while  a  com- 
plete revolution  takes  three  hundred  and  sixty-five  days. 
Evidently,  it  is  the  earth's  rotation  which  gives  us  day  and 
night,  while  its  revolution  determines  the  length  of  the 


FIG.  60. — The  solar  system 


THE  SEASONS  AND  THE  SOLAR  SYSTEM       171 

year.  Let  us  remember,  then,  that  as  the  earth  makes  its 
annual  revolution  about  the  sun,  it  spins  as  it  goes,  and 
each  spin  is  what  we  call  a  day.  Since  the  sun  "rises" 
in  the  east  and  "sets"  in  the  west,  the  direction  of  this 
spinning  is  evidently  from  west  to  east.  At  night  we 
are  in  the  shadow  of  the  earth,  but  we  go  on  spuming  toward 
the  east,  till  presently  the  sun  peeps  over  the  eastern  hori- 
zon, and  we  say  it  is  day. 


FIG.  61. — Diagram  of  the  earth's  orbit. 

The  fact  just  suggested,  namely,  that  earth  and  sun  are 
not  always  the  same  distance  apart,  seems  to  give  us  a 
clew  to  the  solution  of  the  problem  we  are  studying,  the 
problem  of  variations  in  the  amount  of  heat  received  on 
the  earth.  Consider  then  the  shape  of  the  orbit  of  the 
earth  (see  Fig.  61).  It  is  not  a  circle.  It  is  a  kind  of  curve 
called  an  ellipse,  and  when  the  earth,  following  this  path,  is 
farthest  from  the  sun  it  is  three  million  miles  farther  away 
than  when  it  is  nearest.  However,  the  difference  of  dis- 
tance is  so  small  as  compared  with  the  total  distance  that 
evidently  it  can  have  but  small  effect  on  the  amount  of 
heat  received  on  the  earth. 


1 72  ELEMENTARY  SCIENCE 

So  this  clew  alone  does  not  take  us  far.  However, 
in  considering  it,  you  may  have  noted  that  the  figure 
indicates  that  the  axis  of  the  earth  is  inclined.  That  is,  as  it 
spins,  its  axis  does  not  intercept  its  orbit  at  right  angles, 
but  at  an  acute  angle,  and  the  position  of  the  axis  is  the  same 
throughout  the  year.  Here  is  another  clew  we  should 
use,  for  evidently  this  fact  must  have  a  great  effect  upon 
the  way  in  which  the  earth's  surface  intercepts  the  sun's  rays, 
and,  consequently,  upon  the  distribution  there  of  solar 
light  and  heat. 

Thus  we  have  three  great  geometrical  facts  to  consider 
in  studying  the  distribution  of  solar  effects  upon  the  earth. 
These  are  its  revolution,  its  rotation,  and  the  inclination  of 
its  axis.  Evidently,  mathematics  has  an  important  part 
in  the  study  of  nature.  The  problem  we  are  now  consider- 
ing is  largely  a  problem  in  geometry,  and  the  whole  science 
of  the  heavenly  bodies  (astronomy)  is  largely  a  matter  of 
mathematics. 

Effects  of  the  Inclination  of  Earth's  Axis.  —  Let  us  re- 
member that,  in  spite  of  the  complication  of  our  problems 
by  this  newly  observed  fact,  the  fact  that  one-half  of  the  earth 
is  always  in  the  sunshine  remains  the  same.  The  inclina- 
tion of  the  axis  simply  results  in  such  shifting  about  of 
this  sunlit  area  that  we  are  sure  to  be  puzzled  by  it  unless 
we  think  it  out  carefully. 

Now  you  know  that  the  sun  at  noon  in  summer  is  much 
higher  in  the  sky  than  it  is  at  noon  in  winter;  you  know 
that  on  a  winter's  noon  the  sun  stands  considerably  south 
of  the  zenith,  by  which  we  mean  the  point  in  the  heavens 
directly  above  your  head  wherever  you  happen  to  be 
(see  Figs.  62  and  63).  This  evidently  indicates  that  in  our 


THE  SEASONS  AND  THE  SOLAR  SYSTEM       173 

winter  the  north  pole  is  tipped  (inclined)  away  from  the 
sun,  while  in  our  summer  it  is  tipped  toward  the  sun;  in 
the  southern  hemisphere,  the  reverse  is  true.  Thus  we 
see  not  only  why  the  sun  gets  higher  at  noon  in  summer 
than  in  winter;  we  also  see  why  the  sun  in  summer  remains 
visible  longer  than  in  winter.  Since  in  summer  we  are 
tipped  toward  the  sun,  while  in  winter  we  are  tipped  away 
from  it,  of  course  we  see  it  longer  in  summer.  Evidently, 


FIG.  62. — Diagram  to  show  the  direc-  FIG.  63. — Diagram  to  show  the  direc- 
•  tion  of  the  sun's  rays  in  summer;  the  tion  of  the  sun's  rays  in  winter;  the 
north  pole  is  at  the  top.  north  pole  is  at  the  top. 

then,  we  haw  found  out  why  the  days  of  summer  are  longer 
than  those  of  winter.  We  have  found  the  cause  of  the  sea- 
sons. 

The  Equinoxes.  —  Midway  between  the  shortest  and 
longest,  days  of  the  year  come  days  when  it  is  just  twelve 
hours  from  sunrise  to  sunset.  These  days  are  March 
21  and  September  22.  On  these  days,  and  these  days 
only,  the  circle  which  bounds  the  sunlit  half  of  the  earth 
passes  through  both  poles.  So  all  over  the  world,  on  these 
dates,  day  and  night  are  of  equal  length.  These  days  are 
called  the  equinoxes  (aequus,  equal;  nox,  night);  the  one 
in  March  is  called  the  vernal  (per,  spring)  equinox;  the 
one  in  September,  the  autumnal  equinox.  Perhaps  you 


174 


ELEMENTARY  SCIENCE 


have  heard  of  equinoctial  storms.  The  equinoxes  come 
at  stormy  times  of  year  and  it  was  formerly  believed  that 
the  equinox  was  the  cause  of  these  storms.  It  is  now 
known  that  this  is  not  the  case,  but  the  term  is  still 
used. 

The  shortest  day  of  the  year  in  the  northern  hemisphere 
is  evidently  the  longest  day  of  the  year  in  the  southern 
hemisphere;  and  the  reverse  is  also  true.  But,  as  you  have 
noted,  the  equinoxes  come  at  the  same  time  in  both  hemi- 
spheres. Also,  you  should  note,  that  precisely  at  the 
equator,  days  and  nights  are  always  of  equal  length. 
Variations  in  length  of  day  and  night,  and  the  consequent 
effects,  evidently  increase  in  proportion  to  the  distance 
from  the  equator,  until  at  last,  precisely  at  the  poles, 
variation  ceases,  and  day  and  night  are  each  a  half  year 
lon'g.  In  the  tropics,  however,  day  and  night  are  always 
of  nearly  equal  length,  and  there  is  little  variation  in  the 
angle  at  which  the  sun's  rays  strike  the  earth.  Hence  we 
find  in  this  region  much  less  variation  in  the  seasons  than 
we  do  to  the  north  or  south  of  them;  the  farther  we  go 
from  the  equator,  the  more  marked  the  differences  be- 
tween the  seasons  become,  until  we  reach  the  regions  of 
perpetual  ice  and  snow.  Toward  the  poles  the  seasons 
become  merged  in  one  long  winter,  while  toward  the 
equator  they  merge  in  one  long  summer. 

The  Zones.  —  Doubtless  you  learned  hi  geography  that 
the  earth  is  measured  by  two  sets  of  lines  which  are  indi- 
cated on  all  maps,  and  which  give  a  geometrical  method 
of  locating  places.  The  lines  which  run  parallel  to  the 
equator  are  called  parallels  or  lines  of  latitude;  the  ones 
which  cut  these  at  right  angles  and  converge  at  the  poles 


THE  SEASONS  AND  THE  SOLAR  SYSTEM       175 

are  called  meridians  or  lines  of  longitude;  both  parallels 
and  meridians  are,  of  course,  circular  when  complete,  as 
on  a  globe.  A  degree  (symbol,  °  )  is  the  unit  of  length  of 
these  lines.  It  is  Keo  of  their  total  circumference.  The 
degrees  are  subdivided  into  minutes  (symbol,  '),  each  of 
which  is  %o  of  a  degree  (see  Fig.  64). 

Now  the  northernmost  parallel  at  which  (on  account 
of  the  inclination  of  the  earth's  axis)  the  sun  ever  reaches 


FIG.  64. — Parallels  and  meridians. 

the  zenith  is  called  the  tropic  of  Cancer;  the  correspond- 
ing parallel  of  the  southern  hemisphere  is  called  the  tropic 
of  Capricorn.  These  parallels  are,  of  course,  equidistant 
(23°  27'  +  )  from  the  equator,  and  the  belt  of  the  earth 
which  lies  between  them  is  called  the  tropical  zone.  You 
see  then  that  the  sun  seems  to  oscillate  from  one  edge  to 
the  other  of  that  belt  of  the  heavens  which  lies  directly 
over  the  tropical  zone.  Actually,  however,  it  is  not  the 
sun  that  thus  reels  through  the  heavens;  it  is  the  earth 
that  reels.  When  a  moving  train  passes  closely  by  a 
standing  one  in  which  you  are  seated,  you  get  the  im- 
pression that  it  is  yours  which  is  moving.  Similarly  we 


176  ELEMENTARY  SCIENCE 

gain  false  impressions  about  the  relations  as  to  move- 
ment between  earth  and  sun. 

During  that  day,  December  22,  when  the  sun  is  over 
the  tropic  of  Capricorn,  it  is  evident  that  the  region  (cir- 
cular in  outline)  around  the  north  pole  which  remains  in 
darkness  throughout  the  twenty-four  hours  attains  its 
largest  circumference,  for  on  that  day  the  north  pole  is  the 
farthest  within  the  shadow  of  earth  that  it  ever  gets  (see 

Fig.  65).  That  parallel 
which  throughout  that  day 
divides  the  northern  area  of 
darkness  from  the  southern 
*•  area  of  light,  the  last  par- 
allel toward  the  north 
which  the  sun's  rays  touch 
when  it  is  thus  in  its  most 

FIG.   65.— Diagram   showing  where   the      Southerly    position,     that 

^ns  ffSTJSS    parallel  forms  what  we  call 
the  tropk  °f  Cancer  in     the  arctic  circle.    Similarly, 
the  limits  of  illumination, 

when  the  sun  is  at  its  farthest  northern  point,  mark  the 
antarctic  circle.  It  is  a  simple  problem  in  geometry  to 
show  that  the  arctic  circles  must  be  just  as  far  from  the 
poles  as  the  tropics  of  Cancer  and  Capricorn  are  from 
the  equator. 

Between  the  tropics  and  the  arctics  lie  the  great  tem- 
perate zones,  and  it  is  in  these  that  the  seasons  are 
most  marked.  In  them  all  life  responds  to  the  changes 
of  heat  and  cold,  of  rain  or  drought  that  accompany 
the  seasons,  and  so  closely  interwoven  are  these  causes 
and  their  effects  that  we  have  come  to  regard  the  seasonal 
changes  in  plant  and  animal  life  as  parts  themselves  of 


THE  SEASONS  AND  THE  SOLAR  SYSTEM      177 

spring  and  summer,  of  autumn  and  of  winter.  What  are 
some  of  the  things  that  we  call  "signs  of  spring,"  or  "signs 
of  whiter"? 

QUESTIONS 

1.  Why  is  our  longest  day  not  also  our  hottest  day? 

2.  What  is  the  "  solar  system  "? 

3.  Describe  the  movements  of  the  earth,  and  explain  what  rela- 

tion they  have  to  days  and  years. 

4.  In  what  direction  does  the  earth  rotate? 

5.  What  is  the  cause  of  the  seasons? 

6.  What  are  the  equinoxes? 

7.  What  are  parallels  and  meridians? 

8.  What  are  the  tropics?  the  arctic  and  antarctic  circles? 


CHAPTER  XXIV 
WINDS  AND  WEATHER 

In  the  phenomena  we  have  studied  thus  far  the  rela- 
tions between  causes  and  effects  have  been  rather  simple. 
They  have  been  cases  in  which  certain  causes  produce 
certain  effects  which  are  not  difficult  to  predict.  But 
with  wind  and  weather  we  shall  find  that  it  is  different. 
In  fact  you  already  know  that  weather  is  a  difficult  thing 
to  predict,  for  it  is  a  result  not  of  one  but  of  many  causes. 
It  is  so  important  to  man  to  know  what  the  weather  is 
going  to  be  that  millions  of  dollars  are  spent  by  govern- 
ments each  year  in  doing  all  that  science  can  do  to  foretell 
it.  Yet  their  predictions  often  go  wrong. 

The  work  of  a  weather  bureau  consists,  first,  in  securing 
daily  reports  from  all  over  the  country  as  to  local  weather 
conditions.  Then,  with  such  data  in  hand,  it  is  possible 
to  make  forecasts  (predictions)  which  are  fairly  accurate. 
For  weather,  like  all  other  phenomena  of  nature,  is  not 
accidental,  it  is  the  result  of  certain  definite  causes.  The 
difficulty  in  its  prediction  is  that  these  causes  can  be  ascer- 
tained only  in  a  general  way,  and  local  weather  conditions 
may  be  determined  by  purely  local  causes  which  conflict 
with  the  general  causes.  We  shall  see  first  what  the  gen- 
eral causes  are,  and  then  consider  some  of  the  local  causes. 

Weather  Is  the  State  of  the  Atmosphere.  —  We  speak 
of  it  as  warm  or  cold,  as  wet  or  dry.     If  we  stop  to  consider, 
it  is  evident  that  we  mean  by  this  that  the  atmosphere 
178 


WINDS   AND   WEATHER  179 

about  us  is  warm  or  cold,  wet  or  dry.  So  we  see  that 
amounts  of  heat  and  amounts  of  water  are  the  chief  de- 
terminers of  the  nature  of  the  weather,  the  atmosphere 
being  the  substance  in  which  these  determiners  operate. 
Evidently,  then,  the  study  of  weather  is  a  study  of  local  changes 
in  the  state  of  the  atmosphere  as  to  heat  and  as  to  water. 

How  are  such  changes  brought  about  ?  In  the  first  place, 
there  is  the  warming  of  the  atmosphere  when  the  sun 
shines  on  it,  and  its  cooling  off  (refrigeration)  when  the  sun 
is  not  shining  on  it.  This  is  a  process  which  always  op- 
erates with  reference  to  the  atmosphere  as  a  whole.  But 
evidently  it  seems  frequently  not  to  operate  with  refer- 
ence to  local  conditions,  for  the  sun  often  shines  brightly 
during  our  coldest  days.  So  we  must  look  further. 

Already  you  have  noted  that  unequal  heating  of  the 
atmosphere  causes  atmospheric  movements  (convection 
currents).  You  have  also  noted  that  heat  received  from 
the  sun  may  be  chiefly  absorbed  by  ice  and  snow,  and 
so  have  little  effect  on  the  atmosphere.  These  facts  help 
us  to  understand  why  the  air  may  be  cold  even  in  bright 
sunshine. 

Evidently,  then,  the  state  of  the  weather  is  determined  by 
atmospheric  movements  and  by  local  conditions,  more  than 
by  the  direct  action  of  the  sun,  since  these  evidently  can 
overcome  the  direct  effects  of  the  sun.  This  is  even  more 
evident  when  we  consider  weather  as  to  its  wet  and  dry 
qualities,  for  the  distribution  of  water  by  atmospheric 
movements  is  even  more  familiar  to  us  than  the  distribu- 
tion of  heat.  We  have  visible  evidence  of  water  move- 
ment in  air  whenever  we  see  clouds  moving  across  the  sky. 
Heat  movement  in  air,  though  not  visible,  is  just  as  real 
as  water  movement,  and  when  cold  or  hot  winds  strike 


i8o  ELEMENTARY  SCIENCE 

us  we  are  just  as  conscious  of  its  reality.  Why  is  it,  by 
the  way,  that  winds  nearly  always  feel  cool,  even  though 
the  moving  air  may  be  warmer  or  just  as  warm  as  the  air 
it  replaces  ? 

On  a  weather-map  (see  Fig.  66)  you  will  note  that  cer- 
tain areas  are  marked  "low"  and  others  "high."  This 
refers  to  readings  of  the  barometer.  You  remember  that 
a  low  barometer  indicates  the  coming  of  a  change  of 
weather,  and,  usually,  a  storm.  This  is  very  easy  to 
understand  when  you  remember  that  air  or  any  other 
gas  tends  to  diffuse  equally  in  every  direction.  Evidently, 
this  tendency  of  air  to  diffuse  equally  will  cause  it  to 
move  from  areas  of  high  pressure  to  areas  of  low  pressure. 
For  an  area  of  low  pressure  is  simply  an  area  over  which 
there  is  less  air  than  there  is  over  an  area  of  high  pressure. 

But  what  is  it,  you  ask,  that  causes  air  to  thin  out  in 
certain  places  (low  pressure)  and  to  pile  up  (high  pressure) 
in  others  ?  Heat  is  the  answer.  Variations  in  the  amount 
of  heat.  How  often  in  studying  phenomena  we  come 
back  to  heat  changes  as  the  cause!  In  fact,  nearly  all 
changes  in  the  atmosphere  are  due  to  heat;  heat  is  the 
great  explainer. 

Now  we  might  have  said  that  variations  in  atmospheric 
pressure  are  caused  by  convection  currents.  But  what 
causes  convection  currents?  Heat,  of  course.  These  cur- 
rents, whether  in  water  or  in  air,  are  simply  expressions, 
as  it  were,  of  heat,  tools  in  the  hands  of  the  sun's  energy. 
This  you  learned  when  you  studied  these  currents  (see 
Fig.  2).  You  learned  that  the  greater  amount  of  heat 
received  in  the  equatorial  belt  makes  the  atmospheric 
pressure  there  less  than  it  is  to  north  and  south;  that  is, 
the  greater  heat  near  the  equator  causes  the  air  to  expand 


1 82  ELEMENTARY  SCIENCE 

and  rise,  and  then,  as  it  condenses,  to  flow  away  toward 
the  poles.  This  means  that  there  gets  to  be  actually  less 
air  per  cubic  foot  over  the  hot-belt  than  there  is  over  the 
regions  north  and  south.  So,  in  accordance  with  the  law 
of  diffusion,  or  as  we  say,  to  equalize  the  pressure,  cooler 
air  comes  flowing  in  (at  the  bottom  of  the  atmosphere) 
from  north  and  south.  We  may  call  such  Sowings  of  air 
"convection  currents"  if  we  like,  but  they  are  more  com- 
monly called  winds,  the  winds  which  flow  toward  the 
equator  for  the  cause  just  indicated  being  called  the  trade- 
winds.  They  were  so  called  originally  because  they  were 
of  great  assistance  to  the  trade  carried  on  in  sailing  vessels, 
but  they  might  well  have  been  so  called  because  they  are 
part  of  a  trade  of  air  between  one  region  and  another. 

Have  you  not  noticed  that  a  long  spell  of  hot,  dry  weather 
is  almost  sure  to  be  "broken  up"  by  the  coming  of  wind 
and  rain  ?  In  this  case,  too,  a  sort  of  "  trade  "  occurs.  The 
longer  the  hot  spell  continues,  the  lower  the  pressure  tends 
to  become  until  at  last  it  is  as  though  the  atmosphere 
could  "stand  it  no  longer,"  and  winds,  usually  bringing 
rain,  rush  in  to  balance  the  pressure  again. 

We  have  noted  that  air  temperature  has  a  direct  effect 
upon  air-pressure,  and  that  both  have  great  effects  upon 
the  wind  and  the  weather.  Differences  in  pressure  cause 
air  movements,  air  movements  are  wind,  and  the  winds 
bring  the  rain  or  snow.  So  we  see  in  studying  weather  a 
whole  chain  of  causes  and  effects.  We  also  see,  in  this 
connection,  the  need  the  weather  bureau  has  for  making 
diagrams  to  indicate  each  day  the  states  of  temperature 
and  pressure  all  over  the  country.  For  only  by  means  of 
such  facts  as  these  "weather-maps"  present,  can  the  prob- 
able weather  of  the  following  day  or  week  be  foretold. 


1 84  ELEMENTARY  SCIENCE 

Weather-maps  are  marked  by  two  kinds  of  lines.  One 
kind  records  the  variations  in  temperature  over  the  country; 
the  other  records  the  variations  in  pressure.  This  is  done 
by  tracing  across  an  ordinary  map  lines  which  connect 
points  of  equal  temperature;  or  in  the  case  of  pressure, 
lines  which  connect  points  of  equal  pressure.  Lines  which 
connect  points  of  equal  temperature  are  called  isotherms 
(see  Fig.  67) ;  those  which  connect  points  of  equal  pressure, 
isobars.  The  fact  that  isotherms  and  isobars  do  not  corre- 
spond shows  that  other  factors  as  well  as  heat  affect  the 
distribution  of  pressure.  One  of  these  is  the  fact  that 
water  vapor  is  lighter,  per  cubic  foot,  than  air.  So  the 
more  of  water  vapor  there  is  in  air,  other  things  being  equal, 
the  lower  the  pressure.  Hence  we  may  expect  to  find  a  ten- 
dency for  pressure  to  be  less  over  the  oceans  than  over 
the  continents,  and  this  expectation  is  borne  out  by  a 
computation  of  the  facts  shown  by  Fig.  66.  The  average 
pressure  of  the  land  areas  figures  out  as  more  than  the 
average  pressure  of  water  areas.  But  we  must  remember 
that  the  atmosphere  is  very  mobile  (movable),  and  that  the 
earth  is  constantly  spinning  from  west  to  east.  As  it 
spins,  the  gaseous  outer  part  of  it  (atmosphere)  does  not 
quite  keep  up  with  the  solid  and  fluid  inner  parts,  especially 
in  the  equatorial  region,  where  the  motion  of  rotation  is 
much  faster  than  toward  the  poles.  So  it  does  not  surprise 
us  to  find  that  areas  of  low  or  high  pressure  show  a  general 
tendency  to  move,  and  may  not  lie  over  the  areas  where 
they  originated. 

Also  we  note  in  Fig.  66  that,  as  expected,  the  pressure 
recorded  in  the  equatorial  regions  is  decidedly  less  than 
that  to  north  or  south. 

As   to   temperature,   we  have  noted   that  as  altitude 


WINDS  AND   WEATHER 


185 


(remember  mountain  tops)  and  latitude  increase,  the 
temperature,  generally  speaking,  increases.  So  it  does  not 
surprise  us  to  learn  (see  Fig.  67)  that  the  isotherms  show 
a  general  tendency  to  run  parallel  to  the  parallels  (lines  of 
latitude).  Nor  need  the  bends  (away  from  the  parallels) 
of  the  isotherms  surprise  us  when  we  remember  the  local 
effects  that  land  and  water  and  wind  have  on  temperature. 
As  to  altitude  effects,  these  are  not  shown  on  the  maps. 
Allowances  are  made,  when  the 
reports  are  computed,  so  that 
the  isobars  and  isotherms  are 
drawn  on  the  maps  as  though 
all  the  land  were  at  sea-level. 
This  is  because  altitude  is  so 
local  a  condition  that  to  take 
account  of  it  on  weather-maps 
would  only  confuse  a  record 
which  is  meant  to  be  general. 
Fig.  68  indicates  the  gen- 
eral directions  of  the  winds 

of  earth.  This  diagram  is  not  difficult  to  understand  if 
you  keep  in  mind  a  picture  of  the  rotating  earth,  and 
try  to  realize  the  effects  which  this  rotation  is  certain  to 
produce  upon  the  atmospheric  movements.  At  the  equa- 
tor the  rotation  movement  is  over  a  thousand  miles  an 
hour;  to  north  and  south  it  gradually  diminishes  until 
exactly  at  the  poles  it  is  zero.  Now,  of  course,  the  atmos- 
phere very  nearly  keeps  up,  in  this  rotating  movement, 
with  the  rest  of  earth.  Otherwise  we  should  constantly 
have  storms  of  enormous  violence.  As  it  is,  even  in  hurri- 
canes, the  wind  velocity  rarely  becomes  greater  than  one 
hundred  miles  per  hour.  However,  the  rotation  does 


FIG.  68. — Diagram  showing  the  gen- 
eral direction  of  the  winds  of  the 
earth. 


1 86  ELEMENTARY  SCIENCE 

I 

have  some  effect,  and  it  is  this  effect  which  is  portrayed 
in  Fig.  68.  Remember  (see  Fig.  2)  that  the  general  con- 
vection circulation  of  the  atmosphere  is  poleward  from 
the  equator  for  warmer  air,  and  equatorward  from  north 
and  south  for  cooler  air.  Now  if  it  were  not  for  the 
earth's  rotation  these  great  convection  currents  would 
move  due  north  and  south,  paralleling  the  meridians, 
but,  as  it  is,  the  rotation  of  the  earth  causes  them  to 
bend  (blow)  westward  in  low  latitudes  (where  rotation 
speed  is  greater,  and  the  water  and  land  move  a  little 
faster  than  the  air)  and  eastward  in  high  latitudes.  In 
other  words,  the  atmosphere  is  bent  westward  at  the 
equator,  and  this  bending  has  an  effect  which  extends 
clear  up  to  the  polar  regions.  So  Fig.  68  represents  what 
may  be  called  the  basic  directions  of  the  world's  winds. 
But  the  air  movements  so  determined  are  not  so  strong 
but  that  they  may  be  overcome  by  local  conditions. 
So  they  are  often  so  modified  and  we  may  have  even 
violent  winds  which  blow  in  a  direction  opposite  to  that 
of  the  prevailing  winds.  Inasmuch  as  we  live  in  the  re- 
gion indicated  by  the  upper  eastward-pointing  winds,  we 
should  expect  our  prevailing  winds  to  be  westerly,  or 
southwesterly;  that  is,  from  the  west  or  south  of  west. 
Is  this  the  case  ? 

Judging  from  the  diagrams,  you  would  expect  the  equa- 
torial region  to  be,  comparatively  speaking,  a  region  of 
calms,  the  movement  of  the  air  being  more  upward  than 
horizontal.  This  is  the  case.  There  is  a  zone  of  equatorial 
calms  known  as  the  doldrums. 

Also,  note  that  the  trade-winds  north  of  the  equator 
blow  from  the  northeast,  while  south  of  the  equator  they 
blow  from  the  southeast. 


WINDS  AND  WEATHER  187 

Land  and  Sea  Effects  on  Wind.  —  You  have  learned  that 
land  absorbs  heat  more  readily  than  water,  and  also  gives 
it  off  more  readily.  So,  other  things  being  equal,  land  is 
warmer  than  water  in  summer  and  cooler  than  water  in 
winter.  This  may  produce  a  considerable  effect  upon  air 
movements.  Its  tendency  is  to  cause  air  to  move  land- 
ward in  summer  (to  replace  the  ascending  hot  air  there) 
and  waterward  in  winter.  The  most  striking  case  of  this 
is  the  relation  between  the  great  tropical  peninsula  of 
India  and  the  Indian  Ocean  which  surrounds  it.  Here 
the  prevailing  winds  change  with  the  seasons  in  the  manner 
just  indicated.  In  this  region  these  seasonal  winds  are 
called  the  monsoons.  (The  winds  previously  discussed, 
being  affected  by  the  rotation  of  the  planet,  are  called 
planetary  winds.) 

Change  hi  wind  direction  may  also  be  effected  by  the 
temperature  change  of  night  and  day  between  water  and 
land.  This  effect  is,  of  course,  not  so  wide-spread  as  the 
seasonal  change  in  such  a  case  as  India,  but  it  is  enough 
to  give  us  along  the  shores  of  lakes  and  oceans  frequent 
fluctuations  in  the  direction  of  wind.  It  gives  us  what 
we  call  lake  or  sea  breezes,  or  land-breezes.  During  a 
hot  day  and  evening  the  air  may  move  in  from  water  to 
land  (sea-breeze),  and  then,  toward  morning,  if  the  land 
has  cooled  off  more  than  the  water,  it  may  move  water- 
ward  again  (land-breeze). 

QUESTIONS 

1.  Why  do  we  study  the  weather? 

2.  How  are  weather-maps  made  and  what  do  they  tell  us? 

3.  What  causes  low  pressure  and  high  pressure  in  the  atmosphere? 

4.  What  are  the  trade- winds? 

5.  What  are  isotherms  and  isobars? 


1 88  ELEMENTARY  SCIENCE 

6.  Why  is  atmospheric  pressure  lower  over  oceans  than  over  con- 

tinents ? 

7.  Explain  the  general  direction  of  the  winds  of  the  earth. 

8.  What  are  the  "doldrums"? 

9.  Explain  the  effects  of  land  and  sea  on  winds. 


CHAPTER  XXV 

ECONOMIC  IMPORTANCE  OF  WINDS,  RAINS, 
CYCLONES,  CLOUDS,  TORNADOES 

The  economic  importance  of  anything  means  its  relation- 
ship to  mankind,  especially  in  connection  with  his  material 
interests.  A  thing  which  is  economically  important  may 
be  beneficial  to  man,  or  it  may  be  injurious,  or  it  may  be 
both.  So  it  is  with  winds.  They  do  much  good,  but  they 
may  do  much  harm.  In  both  cases  they  are  of  much 
economic  importance,  and  fortunately,  they  do  more  good 
than  harm. 

Windmills  and  sailing  vessels  are  very  ancient  devices 
whereby  man  takes  advantage  of  wind  to  get  work  done. 
But  these,  and  all  other  devices  in  which  the  relationship 
to  wind  is  under  man's  control,  sink  into  insignificance  as 
compared  with  the  great  natural  effects  of  wind  on  man- 
kind, effects  over  which  he  has  no  control  and  which  do 
not  depend  in  the  least  upon  anything  which  he  has  de- 
vised. Of  these  natural  effects  of  wind,  the  distribution 
of  rainfall  is  the  most  important.  It  is  wind  which  brings 
the  rain.  Could  we  get  along  without  rain? 

Uses  of  Rain  and  Snow.  —  The  great  fundamental  occu- 
pation of  mankind  is  agriculture.  For  agriculture  at  least 
twenty  inches  of  rain  per  year  is  generally  necessary. 
There  are  some  exceptions  to  this,  but  not  many.  It  is 
drought  which  the  farmer  usually  fears  more  than  any  of 
his  other  enemies. 

189 


igo  ELEMENTARY  SCIENCE 

Rain  is  needed,  too,  for  man  and  beast  as  well  as  for  the 
crops.  In  cities  rain  flushes  our  streets,  lays  the  dust, 
freshens  our  lawns,  and  fills  our  cisterns.  Though  we  may 
not  use  rain-water  for  drinking,  we  usually  need  it  for 
washing. 

Snow  as  well  as  rain  is  important  for  agriculture.  A 
mantle  of  snow  protects  winter  crops,  like  winter  wheat, 
which  are  planted  in  fall  and  harvested  in  late  spring  or 
early  summer.  Both  rain  and  snow  are  valuable  aids  to 
health.  They  cleanse  the  air  of  dust  and  bacteria.  Have 
you  never  noticed  how  fresh  and  agreeable  the  air  often  is 
after  a  summer  shower? 

Variation  in  Amount  of  Rainfall.  —  The  amount  of  rain- 
fall, as  you  have  learned  in  geography,  varies  extremely 
in  different  parts  of  the  world.  It  depends,  of  course, 
upon  the  winds,  but  it  also  depends  upon  the  regions  from 
which  the  winds  come,  and  upon  the  conditions  which  they 
encounter  when  they  arrive.  The  only  way  in  which  the 
air  obtains  the  water  which  falls  as  rani  is  by  evaporation 
from  the  land  and  water  surfaces  below.  So,  to  bring 
rain,  a  wind  must  come  from  a  region  where  it  could  be- 
come moisture-laden.  Then,  to  give  this  moisture  up  as 
rain,  it  must  encounter  conditions  which  will  cause  it  to 
condense. 

To  illustrate  this,  let  us  take  the  case  of  the  United 
States.  Our  prevailing  winds  are,  as  you  have  noted, 
from  the  southwest.  They  come  to  the  California  coast 
laden  with  moisture  from  the  Pacific.  In  the  winter 
months  the  California  coast  is  cooler  than  the  Pacific, 
so  in  those  months  these  winds  bring  rain.  The  cool- 
ness which  they  encounter  condenses  their  moisture  into 


IMPORTANCE  OF  WINDS  AND  RAINS          191 

drops  (precipitates  it}.  But  in  the  summer  months  the 
land  is  warmer  than  the  sea,  and  the  winds  generally 
pass  on  without  giving  up  their  moisture.  So  the  coast 
of  California  has  distinct  wet  and  dry  seasons,  just  as  is 
common  in  the  tropics.  Farther  to  the  north,  in  Oregon 
and  Washington,  the  rain  is  more  abundant  throughout 
the  year,  because  the  land  there  remains  comparatively 
cool. 

Soon  these  eastward-blowing  winds  encounter  rising 
altitudes;  they  come  to  the  first  range  of  mountains.  Since 
temperature  falls  as  altitude  rises,  they  may  continue  to 
deliver  their  rain.  So  there  is  plenty  of  rain  and  snow 
west  of  the  Sierras  and  the  Cascades.  But  once  these 
mountain  ranges  are  crossed,  and  the  air  begins  to  descend, 
it  becomes  warmer,  and  so  tends  to  retain  what  moisture 
is  left.  Thus  we  see  why  the  -windward  side  of  high  moun- 
tains gets  more  rainfall  than  the  leeward  side,  and  we  also 
see  why  the  Great  Basin  is  so  arid. 

When  these  winds  after  crossing  the  Great  Basin  come 
to  the  Rocky  Mountains  they  may  again  yield  moisture, 
but  after  they  have  crossed  this  second  range,  again  they 
become  dry,  and  the  land  which  they  then  traverse  (west- 
ern Kansas  and  Nebraska)  is  of  little  agricultural  value. 
But  just  beyond  this,  in  the  Mississippi  basin,  lies  the 
richest  agricultural  region  in  the  world,  a  region  where 
rainfall  is  abundant  and  where  farm  lands  are  of  great 
value.  Why  is  this?  Whence  comes  this  rain?  Surely 
there  is  not  enough  water  left  in  the  winds  which  have 
already  traversed  more  than  half  the  continent.  No, 
the  source  of  the  moisture  which  makes  the  farms  of  our 
Middle  West  so  fertile  is  not  the  Pacific  Ocean.  It  is  the 
Gulf  of  Mexico.  But  to  understand  how  the  winds  blow 


192  ELEMENTARY  SCIENCE 

up  from  the  Gulf  and  bring  these  rains,  it  is  necessary  to 
understand  something  about  cyclones. 

A  Cyclone  Is  a  Wind  Which  Rotates.  —  The  thing  about 
which  it  rotates  is  an  area  on  which  the  atmospheric  pres- 
sure is  either  higher  or  lower  than  it  is  on  the  surrounding 
areas.  When  you  hear  the  word  cyclone  you  usually  think 
of  a  terrific  storm,  but  most  cyclones  are  not  terrific  at  all. 
The  ones  which  are  terrific  are  properly  called  tornadoes 
instead  of  cyclones.  You  have  often  seen  cyclones,  at 
least  in  miniature.  On  hot  days  you  have  seen  little  as- 
cending whirlwinds  of  dust  that  rise,  and  move  along,  and 
get  you  all  dusty  if  they  happen  to  come  your  way.  These 
are  miniature  cyclones. 

Now  the  cause  of  this  miniature  cyclone  is  this.  Some 
of  the  air,  nearest  the  earth,  has  become  heated  (mostly 
by  conduction)  until  it  is  decidedly  lighter  than  the  air 
which  overlies  it.  At  some  point  where  the  heating  is 
particularly  strong,  this  lighter  ah*  begins  to  rise,  and 
breaks  a  passage  through  the  cooler  air  above.  Into  this 
passage  more  of  the  low-lying  hot  air  rushes;  soon  it  is 
ascending  from  the  ground  in  a  whirling  column  which 
carries  dust  up  with  it,  and  moves  along  erratically  before 
the  breeze,  becoming  dissipated  as  its  supply  of  hotter 
air  becomes  exhausted,  and  the  pressures  become  equalized. 

Cyclones  similar  in  principle  to  these  little  whirlwinds 
occur  on  a  great  scale.  They  may  involve  hundreds  or 
even  thousands  of  square  miles,  and  they  may  have  much 
to  do  with  the  distribution  of  rain.  Such  cyclones  center 
upon  areas  of  low  pressure;  air  is  drawn  inward  toward  its 
center  and  there  passes  upward.  So,  wherever  we  see  on 
weather-maps  centers  (areas)  of  low  pressure  with  higher 


IMPORTANCE  OF  WINDS  AND  RAINS 


193 


pressures  all  about  them,  we  may  know  that  there  the  air 
tends  to  circle  about  this  center,  converging  toward  it, 
and  moving  upward  as  it  reaches  the  center.  We  can 
also  understand  that  this  whole  great  rotating  mass  of  air 
tends  to  move  also  in  the  direction  of  the  prevailing  winds 
which  push  upon  it. 

The  arrows  of  Fig.  69  indicate  the  movement  of  air 
in  such  a  cyclone  as  viewed  from  above,  while  those  in 
Fig.  70  indicate 
these  movements  as 
viewed  from  the 
side,  assuming  that 
the  prevailing  winds 
are  from  left  to 
right. 

Another  kind  of 
cyclone  is  the  anti- 
cyclone. Its  center 
is  a  center  of  high 
pressure.  In  it  the 
air  moves  downward 
and  outward  instead 
of  inward  and  up- 
ward. For  the  rea- 
son you  have  already  learned,  you  can  understand  that 
cyclones  in  the  southern  hemisphere  rotate  in  the  reverse 
direction  from  those  in  the  northern  hemisphere;  their 
rotation,  whether  the  movement  be  inward  or  outward, 
is  influenced  by  the  earth's  rotation  as  well  as  by  the  differ- 
ence in  atmospheric  pressure. 

}  It  is  a  frequently  observed  fact  that  rain  or  snow  com- 
monly accompanies  areas  of  low  pressure,  while  clear 


FIG.  6g.—  Diagram  showing  the  direction  of  winds 
around  areas  of  high  (H)  and  low  (L)  pressure, 

when  viewed  from  above.  N  ?nd  s  indicate 
°  S°Uthem    hemispheres--After 


194 


ELEMENTARY  SCIENCE 


weather  commonly  accompanies  areas  of  high  pressure.  In 
view  of  what  has  just  been  stated  you  should  be  able  to  ex- 
plain this  fact.  Remember  that  rising  air  is  cooled  and  so 
tends  to  give  up  moisture,  while  as  to  descending  air,  the 
reverse  is  true. 

Now,  to  go  back  to  our  case  of  the  rains  of  the  Mississippi 
Valley,  it  is  a  fact  that  areas  of  low  pressure  frequently 
originate  in  the  region  of  the  Gulf  of  Mexico,  and,  passing 
across  it,  move  up  the  Mississippi  Valley,  and  then  are 


FIG.  70. — Side  view  of  a  cyclone  as  described  in  the  text. — After  SALISBURY. 

carried  by  the  prevailing  winds  off  to  the  east  and  north- 
east. By  this  means  much  air,  carrying  moisture  from  the 
Gulf,  is  drawn  northward,  and,  as  it  goes,  it  cools  and 
delivers  this  moisture  as  rain.  Can  you  not  see  from  this 
why  it  is  that  an  east  wind,  throughout  the  Mississippi 
Valley,  is  considered  a  ''sign  of  rain"?  The  cyclonic 
winds,  which  have  just  been  described  as  the  principal  rain 
bringers  in  this  region,  first  make  their  presence  felt  by  an 
easterly  or  southeasterly  breeze,  which  gradually  veers 
around  to  the  north  and  northwest  as  the  "storm  center" 
moves  to  the  east. 

Study  of  Weather-Maps.  —  Now  you  should  be  in  a 
position  to  study  and  interpret  such  a  weather-map  as 
that  given  on  page  181,  on  which  isotherms,  isobars,  direc- 


IMPORTANCE  OF  WINDS  AND  RAINS          195 

tion  of  winds,  and  cloudy,  rainy,  and  sunlit  areas  are  all 
indicated.  This  is  the  kind  of  weather-map  which  is  issued 
daily  by  the  government.  Accompanied  by  the  weather 
forecast,  such  maps  are  posted  up  in  post-offices  and  other 
places  all  over  the  country.  You  will  find  it  interesting  and 
instructive  to  examine  and  try  to  interpret  them. 

Thunder-Storms.  —  Have  you  ever  watched  the  approach 
of  a  thunder-storm  ?  On  some  hot  summer  afternoon  you 
may  have  seen  dark  clouds  piling  up  in  the  west,  and 
rapidly  approaching.  They  seem  to  hang  low  over  the 
earth,  and  to  mount  high  in  the  sky,  until  they  may  fill 
half  the  heavens,  and  it  becomes  as  dark  as  twilight.  Then 
comes  a  gust  of  cool  wind,  followed  by  the  first  big  drops. 
It  is  time  now  to  run  for  shelter,  for  the  storm  is  about 
to  "break."  Soon  there  comes  a  great  downrush  of  rain. 
There  are  dazzling  flashes  of  lightning,  and  thunder  like  the 
booming  of  many  cannon.  But  soon  the  rain  stops,  the 
sky  clears,  and  the  clouds,  white  now  with  the  sunlight  on 
them,  float  away  to  the  east.  The  air  is  cooler  and  the 
whole  earth  seems  refreshed.  In  the  eastern  sky  there 
may  be  a  rainbow. 

;  Such  storms  are  usually  quite  local.  They  are  due 
to  what  may  be  called  an  "overload"  of  moisture  in  the 
atmosphere,  from  which  it  finds  relief  through  sudden 
rather  than  through  gradual  means.  Since  the  hotter 
the  air,  the  more  moisture  it  absorbs,  it  is  easy  to  see  why 
thunder-storms  usually  occur  in  summer,  on  particularly 
hot  days,  more  frequently  by  day  than  by  night,  and  in 
the  afternoon  more  frequently  than  in  the  morning.  Light- 
ning is  the  passage  of  strong  currents  of  electricity  through 
the  atmosphere.  Sometimes  these  currents  strike  down  to 


196 


ELEMENTARY  SCIENCE 


earth  and  cause  effects  which  are  familiar.  Heat  lightning 
is  the  reflection  on  clouds  of  lightning  which  occurs  below 
the  horizon.  Thunder  is  caused  by  vibration  of  the  air 
due  to  the  passage  of  lightning  through  it. 


FIG.  71.— Cumulus  clouds. 


FIG.  72. — Cumulo-nimbus  clouds. 


FIG.  73. — Cirrus  clouds.  FIG.  74. — Cirro-stratus  clouds. 

After  Cloud  Chart,  Hydrographic  Office,  Department  of  Navy. 

Kinds  of  Clouds.  —  Cumulus  clouds  are  the  kind  that 
seem  piled  up  in  the  sky,  with  rounded,  swelling  contours 
above,  and  an  almost  horizontal  base  (see  Fig.  71).  They 
are  formed  by  rising  water  vapor,  and  their  level  bases 
indicate  the  height  at  which  this  vapor  condenses  to  such 
a  point  that  it  interferes  with  the  light,  so  that  it  becomes 
visible  as  cloud.  Rain-clouds  are  the  nimbus  kind;  they 


IMPORTANCE  OF  WINDS  AND  RAINS          197 

are  dark  and  of  irregular  shape.  The  clouds  of  thunder- 
storms seem  to  combine  the  qualities  of  both  these  kinds; 
they  are  called  cumulo-nimbus  clouds  (see  Fig.  72). 
Cirrus  clouds  are  feathery  (see  Fig.  73).  They  seem  the 
smallest  clouds,  and  are  highest  in  the  air;  five  miles  or 
more.  Usually  they  occur  in  large  groups.  When  they 
are  abundant,  the  sky  is  said  to  be  a  "mackerel  sky." 
It  is  believed  that  this  kind  of  cloud  is  composed  of  fine 
particles  of  snow  and  ice.  Stratus  clouds  are  the  low-lying, 
horizontal  clouds  that  seem  to  lie  in  layers  (see  Fig.  74). 
Of  course  there  are  many  gradations  between  these  various 
kinds  of  clouds,  which  really  merge  one  into  the  other. 

Tornadoes.  —  These  destructive  storms,  usually  called 
cyclones,  are  due  to  very  strong  convection  currents  with 
very  small  diameters.  They  are  like  the  little  dust  whirl- 
winds, multiplied  millions  of  tunes  in  power.  Like  the 
little  whirlwinds,  a  tornado  cloud  has  a  funnel  shape  which 
spreads  out  more  or  less  widely  at  the  top.  In  the  center 
of  this  funnel,  due  to  the  great  uprush  of  air,  the  atmos- 
pheric pressure  may  be  much  less  than  normal  atmospheric 
pressure.  So  we  see  why  houses  in  the  path  of  a  tornado 
sometimes  "blow  up"  from  the  inside.  It  is  because  the 
pressure  of  the  outer  air  is  temporarily  removed,  and  the 
inner  air  in  consequence  expands  with  such  violence  as  to 
rend  walls  asunder.  Here  is  an  excellent  proof  of  the  real- 
ity of  this  atmospheric  pressure  which  we  do  not  actually 
feel. 

Tornadoes  are  rarely  more  than  a  thousand  feet  in  diam- 
eter at  the  bottom,  usually  much  less.  So  we  can  see  why 
the  path  of  destruction  which  they  leave  in  their  wake  is 
narrow.  Within  this  path,  however,  judged  by  the  damage 


198  ELEMENTARY  SCIENCE 

done,  the  wind  may  reach  a  velocity  of  four  or  five  hundred 
miles  per  hour. 

QUESTIONS 

1.  Why  are  rain  and  snow  useful  to  man? 

2.  Why  is  there  more  rain  in  Oregon  and  Washington  than  in 

California? 

3.  Why  is  the  Great  Basin  so  dry? 

4.  Explain  cyclones. 

5.  Explain  the  relation  of  rain  to  areas  of  low  pressure  and  high 

pressure. 

6.  Why  is  there  so  much  rain  in  the  Mississippi  Valley? 

7.  What  is  heat  lightning? 

8.  What  are  the  various  kinds  of  clouds? 

9.  What  are  tornadoes? 


CHAPTER  XXVI 
HEATING  AND  VENTILATION 

We  have  been  considering  the  relations  between  atmos- 
phere and  man  out-of-doors.  But  since  most  of  us  spend 
much  of  our  lives  indoors,  it  is  important  also  to  consider 
the  atmosphere  in  its  relation  to  our  indoor  life. 

Houses.  —  Houses  are  an  excellent  invention.  They 
and  our  clothing  are  the  principal  means  whereby  we  satisfy 
the  shelter  need.  It  is  difficult  to  tell  which  of  these 
shelter  providers  was  invented  first.  Primitive  man  prob- 
ably began  constructing  or  rinding  shelter  among  the  rocks, 
or  among  the  trees  of  the  forest,  just  as  soon  as  he  began 
to  clothe  himself  with  the  skins  of  wild  animals  or  with 
rude  garments  made  from  the  fibrous  parts  of  plants. 

Houses  undoubtedly  provide  excellent  protection  from 
rain  and  wind  and,  usually,  from  cold.  The  only  objec- 
tion to  them  is  that  they  may  interfere  (especially  when 
artificially  heated)  with  the  supply  of  fresh  air  that  we 
need.  So  we  often  hear  it  said  that  people  stay  indoors 
too  much.  We  hear  a  good  deal  about  the  benefits  of 
outdoor  life,  which  means,  besides  exercise,  plenty  of  fresh 
air  and  sunshine.  These  blessings  indoor  life  cannot  pro- 
vide. Since  man  developed  as  an  outdoor  rather  than  as 
an  indoor  creature,  his  body  is  fashioned  for  outdoor  rather 
than  for  indoor  life;  it  pales  and  sickens  if  denied  its 
heritage  of  fresh  air,  exercise,  and  sunshine.  So  there  is 
199 


200  ELEMENTARY  SCIENCE 

virtue  in  sleeping  either  with  the  windows  open,  or  out-of- 
doors.  Many  modern  houses  have  specially  constructed 
sleeping  porches,  and  these  usually  repay  in  health  far  more 
than  they  cost  in  money. 

The  health  problem  in  house  construction  is  evidently 
to  provide  adequate  shelter,  and  at  the  same  time  pro- 
vide as  far  as  possible  the  health  conditions  of  out-of-doors; 
it  is  to  shut  out  the  outdoor  conditions  which  are  disagree- 
able, and  to  admit  those  which  are  agreeable;  it  is  to 
exclude  cold  and  dampness,  and  at  the  same  time  to  admit 
fresh  air.  But  how  are  we  to  shut  out  the  cold,  without 
at  the  same  time  shutting  out  the  air?  They  are,  for 
practical  purposes,  one  and  the  same  thing.  It  is  evidently 
a  rather  complicated  problem.  It  is  the  problem  of  com- 
bining good  heating  with  good  ventilation.  By  ventila- 
tion is  meant  the  delivery  of  fresh  air  and  the  removal 
of  that  which  is  impure.  In  warm  weather,  of  course, 
this  problem  is  simple,  for  then  we  can  leave  the  doors 
and  windows  open  as  much  as  we  like,  closing  them  only 
to  keep  out  rain  and  dust.  But  in  cold  weather  this  prob- 
lem is  always  with  us,  and,  to  get  satisfactory  results, 
every  member  of  the  household  should  understand  thor- 
oughly the  heating  system  which  is  used,  and  the  rules 
which  should  be  observed  in  controlling  it. 

Heating.  —  The  log  cabins  of  the  early  settlers  were 
heated  by  the  burning  of  wood  in  open  fireplaces,  and  it 
was  around  the  hearthstone  that  the  family  life  centered. 
So  the  fireplace  came  to  mean  more  than  merely  a  place 
to  keep  warm.  It  was  a  place  for  rest  and  friendliness;  a 
place  for  story-telling,  and  for  the  entertainment  of  a  guest. 
It  is  a  place  which  seems  to  soothe  the  heart  of  man, 


HEATING  AND  VENTILATION  201 

inviting  friendliness  and  forgiveness,  dismissing  mistrust 
and  hate.  It  is  a  place  which  makes  it  easier  to  tell  one's 
thoughts,  and  people  sitting  together  before  the  embers 
of  a  dying  fire  have  come  to  know  each  other  really  better 
than  in  any  other  place  in  the  world. 

So  it  is  easy  to  see  why  there  is  a  great  deal  of  sentiment 
about  a  fireplace,  and  why  it  is  so  pleasant  a  feature  of  a 
living-room.  But,  however  good  a  fireplace  may  be  for 
friendship  and  sentiment,  it  is  not  very  good  for  practical 
heating  purposes.  It  warms  you  well  enough  on  one  side, 
but  on  the  other  (unless  there  is  some  other  source  of  heat) 
you  are  likely  to  be  cold.  Also,  from  the  merely  practical 
standpoint,  a  fireplace  is  a  great  waster  of  fuel.  Most 
of  the  heat  produced  goes  up  the  chimney.  Yet  even  that 
has  its  advantages,  for  the  warm  and  comparatively  impure 
air  which  rushes  up  the  chimney  is  replaced  by  fresh  air 
drawn  in  from  outside.  Hence  there  is  plenty  of  ventila- 
tion. Too  much  of  it,  if  anything. 

This  suggestion  as  to  waste  of  fuel  indicates  one  of  the 
principal  factors  in  this  problem.  It  is  not  difficult  to 
get  good  ventilation  so  long  as  you  don't  care  how  much 
fuel  you  burn.  If  you  keep  letting  the  heated  air  out  and 
fresh  air  in  you  will  get  plenty  of  ventilation,  but  it  will 
be  expensive  to  keep  the  house  warm  enough.  So  evidently 
the  problem  becomes  to  secure  satisfactory  ventilation  with 
the  least  expenditure  of  fuel.  The  reason  why  many  houses 
and  other  buildings  are  badly  ventilated  is  that  those  in 
charge  are  thinking  more  about  coal  bills  than  they  are 
about  health.  They  care  more  about  keeping  heat  in  than 
about  letting  bad  air  out,  and  if  you  open  a  window  they 
are  apt  to  say:  "Are  you  trying  to  heat  all  outdoors?" 
By  the  way,  in  "airing  out"  a  room,  why  is  it  best  to  open 


;202  ELEMENTARY  SCIENCE 

a  window  both  at  the  top  and  at  the  bottom^thus  providing 
two  openings  ? 

After  fireplaces  came  stoves,  which  are  simply  fire-holders 
so  built  that  the  consumption  of  fuel  is  economical  and 
the  transfer  of  heat  is  into  the  room  rather  than  up  the 
chimney.  An  important  feature  of  the  stove  is  the  draft 
whereby  air  is  admitted  from  below  to  provide  the  oxygen 
necessary  for  the  burning  of  the  fuel;  the  rate  at  which 
the  fire  burns  may  be  controlled  by  the  opening  or  closing 
of  the  draft.  In  the  case  of  fireplaces,  andirons  are  used  to 
secure  the  needed  draft  of  air  from  below.  The  draft 
(drawing)  of  air  through  the  stove  is  also  controlled  by  a 
damper,  which  is  a  movable  plate  by  means  of  which  the 
flue  may  be  open  or  closed. 

The  closing  of  the  flue,  through  which  the  hot  gases 
from  the  fire  flow,  checks  the  draft,  and  so  lowers  the  rate 
of  burning.  It  also  checks  the  heat  from  going  up  the 
chimney.  So  the  damper  is  usually  left  open  while  the 
fire  is  starting,  and  then  closed  or  partly  closed  when  the 
fire  is  burning  well.  We  should  remember  that  wood  and 
coal  in  burning  give  off  gases,  and  it  is  the  burning  of  these 
gases  as  well  as  the  burning  of  solid  fuel  that  produces  the 
heat  we  feel.  So  we  see  that  stoves  have  another  advantage 
over  fireplaces  in  that  they  give  us  the  benefit  of  the  heat 
from  these  burning  gases  more  than  fireplaces  do. 

A  furnace  is  better  than  a  stove  in  that,  by  means  of 
it,  the  heat  of  a  single  fire  may  be  conveyed  over  the 
whole  house.  It  permits  of  greater  economy  in  the  con- 
sumption and  in  the  transportation  of  fuel,  as  well  as 
in  the  distribution  of  heat.  A  furnace,  strictly  speaking, 
is  that  part  of  a  heating  plant  in  which  the  heat  is  gener- 
ated. For  the  conveyance  of  this  heat  to  other  points, 


HEATING  AND  VENTILATION 


203 


various  means  are  used;  this  conveyance  may  be  by  means 
of  currents  of  hot  air,  of  hot  water,  or  of  steam.  (It  is 
common  to  use  the  term  furnace  as  applied  to  hot-air 
plants  only,  but  there  is  no  real  reason  why  the  term  should 
be  so  restricted.) 

The  hot-air  type  is  the  cheapest  type  of  heating  plant  to 
install,  and  has  the  further  advantage  of  distributing  heat 
more  rapidly  than  other 
types.  The  chief  ob- 
jections to  it  are  that 
it  may  also  distribute 
dust,  and  that  it  may 
cause  the  air  to  become 
too  dry  to  be  comfort- 
able. This  latter  ob- 
jection may  be  con- 
siderably offset  by 
keeping  large  pans  of 
water  under  the  reg- 
isters through  which 
the  hot  air  comes  into 
the  rooms.  You  should 
remember  that  as  air  heats  and  expands  it  increases  in 
power  to  absorb  moisture.  So,  whatever  heating  system 
is  used,  provision  needs  to  be  made  for  keeping  the  air 
moist  enough  to  be  comfortable. 

By  studying  Fig.  75  you  can  see  how  a  hot-air  furnace 
works.  However,  it  is  not  quite  as  simple  a  matter  as  it 
seems.  For  one  thing,  there  must  be  careful  figuring  to 
insure  that  enough  fresh  air  enters  the  furnace  to  feed  the 
hot-air  pipes  that  lead  away  from  it.  You  will  note  in 
the  figure  that  only  one  cold-air  flue  enters  the  chamber 


FIG.  75. — Diagram  showing  how  a  hot-air  fur- 
nace works. — After  MILLIKAN  and  GAI.E. 


204 


ELEMENTARY  SCIENCE 


(hot-air  jacket)  which  surrounds  the  fire-box,  while  many 
flues  lead  away  from  it.  Since  air  expands  as  it  is  heated,  it 
is  evident  that  the  area  of  the  cross-section  of  the  feed- 
flue  does  not  have  to  be  so  large  as  the  combined  areas  of 
the  exit-flues;  the  right  difference  between  them  will  be 
the  difference  between  the  volume  of  the 
air  when  it  enters  the  hot-air  jacket  and  its 
volume  when  it  leaves.  Many  hot-air  fur- 
naces do  not  work  well  for  the  reason  that 
the  feed-flue  is  not  large  enough  in  propor- 
tion to  the  exit-flues;  often  the  efficiency 
of  such  a  furnace  will  be  greatly  increased 
by  adding  another  cold-air  flue. 

Then  there  is  the  question  of  whether  the 
air  which  feeds  the  furnace  shall  be  drawn 
from  outdoors  or  indoors.  That  drawn  from 
outdoors  has  the  advantage  of  freshness. 
That  drawn  from  indoors  has  the  advantage 
of  economy;  it  is  already  partly  warmed; 
this  arrangement  has  the  advantage  of  in- 
suring better  circulation  of  air  in  the  rooms 
from  which  it  is  drawn.  Usually  both  of 
these  sources  of  air  are  used;  some  is  drawn  in  from  out- 
doors, and  some  from  the  rooms  above  the  furnace.  This 
is  the  reason  why  you  find  in  some  rooms  both  cold-air  and 
hot-air  registers,  the  use  of  the  former  being  simply  to  fur- 
nish an  exit,  the  draft  being  into  rather  than  out  of  it. 

A  hot-water  heating  system,  like  a  hot-air  system,  depends 
first  on  the  distribution  of  heat  by  convection  currents. 
The  movements  of  convection  currents  of  water  may  be 
nicely  demonstrated  by  means  of  such  an  apparatus  as  that 
shown  in  Fig.  76;  this  shows  clearly  the  principle  of  a  hot- 


Fio.  76. — Diagram 
showing  the  prin- 
ciple of  the  hot- 
water  heating 
system.  Explain 
this. 


HEATING  AND  VENTILATION 


205 


water  heating  apparatus.  But  let  us  note  that,  after  the 
heat  reaches  the  coils  of  pipe  in  the  rooms  above  the 
furnace,  it  is  then  given  off  into  the  room  by  radiation; 
hence  we  call  these  coils  of  pipe  radiators. 

A  hot-water  system  (see  Fig.  77)  requires  that  there  be 
an  open  (expansion)  tank  at  the 
top,  so  that,  if  steam  forms,  it 
may  escape  without  bursting  the 
pipes.  When  it  does  form,  it 
causes  "pounding"  in  the  pipes, 
but  it  should  not  form  if  the 
furnace  is  properly  controlled. 
Hot-water  systems  have  the  ad- 
vantage of  supplying  an  "even" 
heat;  they  also  do  not  cause  so 
much  dryness  of  the  air  as  do 
hot-air  systems. 

Steam-heating  plants  are  oper- 
ated on  the  same  principle  as  hot- 
water  plants,  but  they  require  a 
boiler  in  which  the  water  is  trans- 
formed to  steam,  and  a  pressure- 
gauge  which  must  be  watched  to 
see  that  the  proper  amount  of  pressure  is  maintained. 
They  are  preferred  in  the  heating  of  large  buildings,  or  in 
the  heating  of  several  buildings  by  the  same  heating  plant, 
for  the  reason  that  heat  can  be  transported  through 
pipes,  for  considerable  distances,  by  means  of  steam  more 
successfully  than  by  means  of  hot  air  or  hot  water.  Why 
is  this  so? 

Sometimes  combinations  of  heating  systems  are  used. 
Thus  the  radiators  of  a  hot-water  or  steam  system  may  be 


FIG.  77. — Diagram  of  a  hot- 
water  system. 


206 


ELEMENTARY  SCIENCE 


placed  in  the  basement  in  an  enclosure  through  which  air 
passes  over  them.  This  air,  when  heated,  is  delivered  by 
flues  to  the  rooms  above.  Very  satisfactory  results  may 
be  obtained  by  such  an  indirect  system  (see  Fig.  78). 

Ventilation.  —  You  have  already  noted  that  the  chief 
difficulty  in  ventilation  is  that  it  works  at  cross-purposes 
with  heating;  it  is  hard  to  ventilate  a  building  properly 

and  at  the  same  time  to 
heat  it  sufficiently.  In  late 
years,  however,  it  has  been 
proved  that,  to  make  bad 
air  good,  it  may  not  be  nec- 
essary to  change  it.  In  other 
words,  bad  air  may  be  made 
good  by  washing  it  and  stir- 
ring it.  This  is  done  by 
driving  it  through  a  screen 
of  falling  water,  whereby 
the  disease-germs  and  other 
FIG.  7s.-The  indirect  heating  system,  impurities  are  removed. 

Such  ventilation  systems 

have  not  come  into  general  use,  but  they  are  successfully 
and  economically  operated  in  a  number  of  places. 

There  is  little  doubt  that  the  frequent  colds  that  peo- 
ple catch  in  winter,  and  which  so  frequently  lead  to  seri- 
ous or  even  fatal  illnesses,  are  largely  due  to  poor  ventila- 
tion. In  badly  ventilated  rooms,  especially  in  those  used 
by  the  general  public,  disease-germs  accumulate  and  mul- 
tiply. Good  ventilation  would  drive  them  out,  or  reduce 
their  number  below  the  danger-point.  Bad  ventilation 
particularly  promotes  tuberculosis,  which  is  one  of  the 


HEATING  AND  VENTILATION 


207 


greatest  enemies  of  mankind.  Doubtless  you  have  heard 
of  the  fresh-air  treatment  for  this  disease.  Doctors  insist 
that  tubercular  patients  shall  have  all  the  fresh  air  it  is 
possible  for  them  to  get;  this  increases  their 
power  to  resist  and  overcome  the  disease. 

Ventilation  was  no  problem  at  all  when 
houses  were  loosely  built  and  heated  by 
fireplaces  only.  The  heated  air  which 
rushed  up  the  chimneys  was  quickly  re- 
placed by  fresh  air  which  came  in  under  the 
doors  and  through  other  crevices.  There 
were  plenty  of  "drafts,"  which,  though  un- 
comfortable, probably  did  more  good  than 
harm.  But  now  that  man  lives  mostly  in 
snugly  built  and  usually  overheated  houses, 
he  must  give  serious  attention  to  his  fresh- 
air  supply.  It  is  usually  estimated  that 
about  two  thousand  cubic  feet  of  fresh  air 
per  hour  is  desirable  for  each  individual. 

In  private  houses  ventilation  usually  con- 
sists in  simply  opening  the  windows,  and 
this  may  suffice  if  it  is  done  often  enough, 
and  if  circulation  is  provided  by  also  opening  some  doors. 
But  in  the  case  of  public  buildings,  especially  in  the 
case  of  public  schools,  there  is  need  for  the  installation, 
of  special  systems  of  ventilation.  Very  likely  you.  have, 
noticed  in  your  own  schoolroom  a  large  radiator  in  the 
wall  through  which  fresh  air  blows  into  the  room,  and 
ducts  through  which  bad  air  escapes.  Large  fans  operated 
by  electricity  are  commonly  used  to  drive  the  air  through 
such  ventilating  systems.  Or  the  fans  may  be  used  to 
draw  the  bad  air  out  of  the  rooms.  The  fresh  air  which  is 


FIG.  79.  — The 
thermostat. 


208  ELEMENTARY  SCIENCE 

driven  into  rooms  by  a  ventilating  system  is  warmed  first, 
but  usually  there  is  a  heating  system  which  is  independent 
of  the  ventilating  system,  as  well  as  co-operating  with  it. 
A  thermostat  is  a  device  for  regulating  temperature.  It 
is  a  great  convenience.  You  can  set  the  hand  of  a  thermo- 
stat at  the  desired  temperature,  and  the  heat-supply  will 
be  so  regulated  that  this  temperature  will  be  attained  and 
maintained  as  long  as  there  is  an  adequate  fire  in  the  fur- 
nace. In  the  usual  type  of  thermostat  a  small  metal  rod 
is  attached  to  a  curved  strip  of  metal.  The  end  of  this 
rod  lies  between  two  electric  terminals.  If  the  temperature 
rises  above  what  is  desired,  the  rod,  due  to  the  expansion 
of  the  curved  metal  strip,  at  once  comes  in  contact  with 
one  of  the  electric  terminals,  completing  the  circuit,  and 
the  heat-supply  is  thereby  checked.  If  the  temperature 
falls,  the  other  circuit  is  completed,  and  the  heat-supply  is 
increased  (see  Fig.  79). 

QUESTIONS 

1.  What  is  meant  by  ventilation? 

2.  How  should  a  room  be  aired? 

3.  What  are  the  advantages  and  disadvantages  of  a  fireplace? 

4.  Why  are  stoves  better  than  fireplaces  ? 

5.  What  are  the  advantages  and  disadvantages  of  a  hot-air  furnace  ? 

6.  Why  is  the  hot-water  heating  system  better  than  the  hot-air 

system  ? 

7.  What  advantages  have  steam-heating  plants? 

8.  What  is  the  use  of  a  thermostat? 


CHAPTER  XXVII 
COMBUSTION 

If  you  have  ever  been  the  keeper  of  a  furnace  fire,  you 
know  that  "firing"  is  not  so  easy  as  it  seems.  There  is 
a  good  deal  more  to  it  than  simply  throwing  in  the  coal  and 
taking  out  the  ashes.  The  fireman  of  a  locomotive  or  of 
a  steam-power  plant,  if  he  is  efficient,  must  understand 
how  to  fire  so  as  to  secure  the  maximum  of  heat  with  the 
minimum  consumption  of  fuel.  This  is  a  rather  compli- 
cated matter  and  requires  a  thorough  understanding  of  the 
laws  of  combustion. 

There  are  various  kinds  of  combustion.  Burning  is  one 
kind.  Rusting  is  another.  The  combustion  of  food 
within  our  bodies  is  yet  another.  Evidently,  if  these 
phenomena  are  examples  of  it,  it  must  be  a  process  which 
is  important  to  understand. 

What  Combustion  Is.  —  Combustion  is  the  chemical 
uniting  of  a  combustible  substance  with  oxygen.  It  names 
a  process  which  you  have  already  considered  under  the 
name  of  oxidation.  Oxidation  names  this  process  from 
the  standpoint  of  oxygen;  combustion  names  it  rather  from 
the  standpoint  of  the  substance  which  is  oxidized. 

That  combustion  is  so  common  a  process  is  due  to  the 

commonness  of  what  we  may  call ' '  oxygen-hunger."    Many 

familiar  substances  show  a  sort  of  eagerness  to  change  by 

uniting  with  oxygen  whenever  they  have  a  chance.    Since 

209 


2io  ELEMENTARY  SCIENCE 

oxygen  is  a  part  of  air,  they  evidently  have  plenty  of  chance 
to  do  this  so  far  as  the  mere  presence  of  oxygen  is  con- 
cerned. But,  fortunately  for  us,  the  combustion  of  most 
inflammable  substances  requires  more  than  the  mere  pres- 
ence of  air;  it  requires  also  more  heat  than  is  ordinarily 
present.  Once  the  process  is  started,  however,  it  may  of 
itself  generate  enough  heat  to  keep  going  until  all  the 
near-by  combustible  material  has  been  consumed.  All  the 
fire  departments  and  fire-insurance  companies  in  the 
world  are  results  of  this  simple  fact.  It  explains  why, 
when  a  fire  gets  started  where  we  do  not  want  it,  we  do 
our  best  to  put  it  out. 

You  remember  that  the  early  scientists  believed  that 
fire  was  due  to  the  escape  from  burning  substances  of  a 
mysterious  and  invisible  substance  they  called  phlogis- 
ton. This  phlogiston  theory,  absurd  as  it  may  seem  now, 
has  an  important  place  in  the  history  of  science.  It  was 
only  a  little  more  than  a  hundred  years  ago  that  it  was 
abandoned  hi  favor  of  the  modern  theory  of  combustion. 

The  early  scientists,  though  they  misinterpreted  the 
process  of  burning,  did  recognize  that  it  and  the  process  of 
rusting  are  similar  processes.  Iron-rust  is  iron  plus  oxygen 
in  the  form  of  a  substance  called  iron  oxide.  This  substance 
flakes  away,  and  may  finally  disappear  as  dust,  so  we  are 
apt  to  regard  rusting  as  a  process  in  which  something  is 
taken  away  from  the  iron.  The  iron  does  actually  decay, 
but  the  thing  for  us  to  remember  is  that  rusting,  and  much 
of  what  we  call  decay,  are  processes  of  oxidation,  and  that, 
as  such,  they  involve  the  addition  of  something  rather  than 
the  subtraction  of  something.  In  all  kinds  of  combustion 
the  resulting  substances  may  be  or  become  gaseous  or 
liquid  (iron-rust  dissolves)  and  disappear,  but  the  fact 


COMBUSTION  211 

remains  that  the  weight  of  all  the  substances  resulting  from 
combustion  is  exactly  equal  to  the  weight  of  the  original  sub- 
stance plus  the  oxygen  which  has  united  with  it. 

Here,  then,  we  have  a  statement  of  the  modern  explana- 
tion of  combustion.  It  is  an  explanation  which  rests  upon 
good  evidence,  for  the  products  of  combustion  have  repeat- 
edly been  carefully  collected  and  weighed  with  results  as 
indicated.  You  have  already  learned  in  your  study  of  air 
that  gas  has  weight  which  can  be  measured.  So  the 
gaseous,  liquid,  and  solid  substances  which  result  from 
combustion,  when  all  weighed,  and  their  weights  added, 
have  always  been  found  to  exceed  the  weight  of  the  original 
substance,  the  difference  being  the  weight  of  the  oxygen 
which  has  been  taken  on. 

Inasmuch  as  the  proof  of  this  explanation  of  combustion 
depends  upon  very  careful  weighings,  it  does  not  surprise 
you  to  learn  that  this  proof  was  one  of  the  results  which 
followed  the  perfection  of  balance  scales,  scales  so  perfected 
that  they  record  the  most  delicate  differences  in  weight. 
If  the  older  scientists  had  had  better  scales,  the  right 
explanation  of  combustion  would  doubtless  be  much  older 
than  it  is.  So  it  has  been  with  many  of  the  greatest  dis- 
coveries of  science;  they  have  resulted  from  the  perfection 
of  some  mechanical  device  which  enables  man  to  get  more 
evidence  than  he  could  get  before.  Thus  the  develop- 
ment of  modern  biology  had  to  await  the  perfection  of  the 
microscope.  To  the  physicist  and  chemist,  delicate  bal- 
ances are  quite  as  important  as  the  microscope  is  to  the 
biologist  or  the  telescope  is  to  the  astronomer.  One  of 
the  first  discoveries  made  by  the  use  of  a  delicate  balance 
was  that  metal  oxides,  such  as  iron  oxide,  weigh  more  than 
the  metals  from  which  they  are  formed.  In  the  light  of 


212  ELEMENTARY  SCIENCE 

this  new  evidence,  the  phlogiston  theory  was  exploded,  and 
the  rest  was  comparatively  easy. 

Taking  Care  of  a  Furnace  Fire.  —  Now  to  go  back  to 
the  matter  of  taking  care  of  a  furnace  fire.  Two  important 
things  to  accomplish  are  to  get  as  much  heat  as  possible 
out  of  the  fuel  used,  and  to  prevent  the  formation  of 
clinkers.  These  two  things  go  together;  you  cannot 
succeed  in  the  first  if  you  fail  to  succeed  in  the  second.  A 
clinker  is  a  mass  which  results  from  the  fusion  of  coal 
which  has  been  only  partly  burned;  by  its  fusion  it  is 
so  changed  that  it  will  not  burn  any  more.  This 
fusion  of  coal  indicates  that,  due  to  overheating,  coal  may 
actually  melt  and  run  together  before  it  burns  up;  the  fused 
masses  (clinkers)  which  result  from  this  are  useless  for  fuel, 
hinder  the  shaking  down  of  ashes,  and  interfere  with  the 
draft. 

So  we  see  that  to  get  the  most  heat  out  of  coal  we  must 
not  try  to  get  it  too  fast;  we  must  nurse  the  fire  along; 
there  is  such  a  thing  as  giving  it  too  much  draft.  The 
aim  should  be  to  provide  a  good  circulation  of  air  through 
the  whole  burning  mass,  so  that  it  may  become  thor- 
oughly oxidized  in  all  its  parts,  and  not  fuse.  Hence  you 
may  spoil  your  fire  by  giving  it  too  much  air  from  below; 
it  is  rather  dangerous  to  leave  the  ash-door  open.  At  the 
same  time,  this  risk  of  too  much  air  from  below  gives  no 
excuse  for  letting  the  ashes  accumulate.  This  will  pre- 
vent access  of  air  evenly  to  all  parts  of  the  fire;  also,  if 
the  heap  of  ashes  rises  till  it  touches  the  grate-bars,  it  will 
(by  adding  to  their  heat  through  conduction)  cause  them 
to  "burn  out"  (melt).  Another  thing  which  may  cause 
clinkers  is  too  rapid  checking  of  the  fire.  When  you 


COMBUSTION  213 

close  the  draft,  the  check-damper  should  not  be  opened  at 
the  same  time;  not  until  a  little  later.  For  too  rapid  cooling 
off  of  the  fire  may  also  cause  fusion.  Another  cause  of 
clinkers  is  poor  coal;  sometimes  you  get  coal  which  is  so 
poor  that  even  the  most  intelligent  firing  will  not  prevent 
clinkering.  In  such  cases  the  coal-dealer  should  be  com- 
pelled to  give  a  rebate,  or  even  to  take  the  coal  away. 

It  is  evident  that  a  good  deal  may  be  learned  from  taking 
care  of  a  furnace.  Just  as  a  boy  who  takes  care  of  a  lawn 
intelligently  will  learn  a  good  deal  about  botany,  so,  in 
taking  care  of  a  furnace,  he  has  an  excellent  opportunity 
to  learn  something  about  physics  and  chemistry. 

Some  of  the  Products  of  Combustion.  —  Let  us  consider 
now  what  are  some  of  the  products  of  combustion.  This 
depends  entirely,  of  course,  upon  what  it  is  that  is  being 
oxidized.  You  can  get  no  products  except  those  that 
result  from  the  combination  with  oxygen  of  the  materials 
which  are  already  present.  In  the  rusting  of  iron,  you  get 
only  iron  oxide.  In  the  burning  of  wood  or  coal,  you  get 
a  number  of  products;  while  in  the  combustion  of  sub- 
stances within  living  bodies,  you  get  an  even  greater  variety. 

Take  the  burning  of  a  candle.  By  holding  a  saucer  in 
the  flame  you  can  collect  soot.  Soot  is  carbon.  It  burns 
in  the  flame  and  becomes  carbon  dioxide.  If  you  collect 
the  gas  given  off  from  a  burning  candle,  or  from  any  other 
kind  of  burning,  you  will  find  that  it  is  largely  made  up 
of  carbon  dioxide.  Smoke  is  unconsumed  carbon.  The 
more  smoke  that  a  fire  gives  off,  the  less  efficient  it 
is  in  consuming  its  fuel;  smoke  indicates  that  some 
carbon,  before  it  is  oxidized,  gets  away  from  the  range 
of  sufficient  heat  to  oxidize  it.  So  smoke  should  be, 


214  ELEMENTARY  SCIENCE 

suppressed,  not  only  because  it  is  a  nuisance,  but  also 
because  such  suppression  means  greater  economy  in  fuel 
consumption. 

By  holding  a  cold  piece  of  iron  in  the  flame,  and  re- 
moving it  before  it  gets  too  hot,  you  can  collect  drops  of 
water.  This  indicates  that  the  candle  contains  hydrogen 
as  well  as  oxygen,  and  that  this  hydrogen  is  being  oxi- 
dized, thus  forming  water.  Of  course  the  heat  causes  this 
water  to  be  given  off  as  invisible  water  vapor. 

Old-fashioned  candles  were  made  of  tallow,  which  was 
derived  from  animal  fat.  The  molecules  of  fat  are  com- 
posed of  just  three  elements:  carbon,  hydrogen,  and 
oxygen.  Modern  candles  are  usually  made  from  substances 
derived  from  petroleum,  but,  like  tallow,  these  substances 
are  composed  of  carbon,  hydrogen,  and  oxygen  only.  So 
evidently  when  we  have  collected  carbon,  carbon  dioxide, 
and  water  from  the  burning  of  a  candle,  we  have  accounted 
for  all  the  substances  which  compose  it. 

The  flame  of  a  candle  is  cone-shaped.  The  center  of 
this  cone,  as  you  have  proved  by  collecting  carbon  from  it, 
is  composed  of  substance  transformed  by  heat  into  a  gaseous 
state,  but  not  yet  completely  oxidized.  It  is  oxidized 
when  it  reaches  the  surface  of  the  cone,  the  part  which  is 
luminous.  You  can  prove  the  presence  of  unconsumed 
gases  in  the  center  of  a  candle  flame  by  introducing  there 
the  end  of  a  small  glass  tube.  If  you  hold  a  match  at 
the  other  end  of  this  tube,  a  flame  will  appear. 

Energy  Released  by  Combustion.  —  Thus  far  we  have 
discussed  only  the  substances  resulting  from  combustion. 
But  we  are  also  familiar  with  the  fact  that,  in  combustion, 
energy  is  released.  This  energy,  in  cases  of  burning,  we 


COMBUSTION 


215 


perceive  in  the  forms  of  light  and  heat.  In  the  rusting 
of  iron  we  do  not  perceive  it;  it  is  too  small  in  amount  at 
any  given  time  to  be  perceptible  to  our  senses,  yet  it  is 
measurable  by  means  of  instruments  of  precision  which 
are  used  in  physical  laboratories.  In  the  case  of  our  own 
bodies,  combustion  releases  energy  which  is  manifested 
both  by  the  heat  of  our  bodies  and  by  the  motions  of  our 
muscles. 

It  may  occur  to  you  that  this  energy  release  should  in 
some  way  confuse  the  results  obtained  by  weighing  the 
substances  which  result  from  combustion  and  comparing 
their  weight  with  that  of  the  original  substance.  But 
let  us  remember  that  energy  is  not  a  substance  and  cannot 
be  weighed.  Let  us  also  note  that  we  speak  of  the  re- 
lease of  this  energy  rather  than  of  its  production,  for  energy, 
as  you  know,  cannot  be  produced;  it  can  only  be  trans- 
formed. So  combustion  is  a  process  of  energy  transforma- 
tion as  well  as  of  substance  alteration.  The  energy  which 
was  confined  or  "stored  up"  in  the  combustible  substance 
is  released,  and  we  may  perceive  it  as  light,  heat,  or  motion. 

QUESTIONS 

1.  What  is  combustion? 

2.  What  is  rusting? 

3.  Does  rust  weigh  more  or  less  than  the  iron  which  formed  it? 

4.  What  are  clinkers? 

5.  What  are  smoke  and  soot  ? 

6.  Is  smoke  necessary? 

7.  What  chemical  elements  are  involved  when  a  candle  burns? 

8.  What  transformation  of  energy  takes  place  when  wood  burns? 


CHAPTER  XXVIII 
LIGHT,  COLOR,  AND  SOUND 

We  have  noted  that  combustion'  may  result  in  light, 
heat,  or  motion,  or  in  combinations  of  these.  To  heat 
and  motion  we  have  already  given  considerable  attention. 
It  is  time  now  to  study  light  and  to  learn  something  of 
its  nature  and  behavior. 

Light  and  Heat  Much  Alike.  —  The  first  thing  for  us  to 
appreciate  is  that  light  and  heat  are  much  alike.  They 
are  both  forms  of  radiant  energy.  Their  apparent  great 
differences  are  due  to  the  differences  in  the  effects  they 
produce  on  us. 

This  sameness-of-nature  or  kinship  of  light  and  heat  is 
indicated  by  many  familiar  phenomena.  We  know  that 
heat  may  be  very  readily  transformed  into  light;  similarly, 
light  may  be  transformed  into  heat.  We  know  that 
white  clothes  are  cooler  than  dark  clothes,  even  though 
of  the  same  weight;  this  is  because  white  clothes  reflect 
the  light,  while  dark  clothes  absorb  it,  and  transform  part  of 
it  into  heat.  We  know  that  sunlight  striking  through 
glass,  as  in  greenhouses,  causes  increase  of  temperature 
even  though  the  air  outside  may  be  cold;  this  is  because 
the  light-rays  are  absorbed  after  they  pass  through  the 
glass,  some  of  them  are  transformed  into  heat,  and  the 
heat-rays  cannot  escape  by  passing  through  the  glass  as 
the  light-rays  did.  We  know  that  in  burning,  both  light 
and  heat  usually  appear,  and  it  is  hard  to  tell  where  one 
216 


LIGHT,  COLOR,  AND  SOUND  217 

stops  and  the  other  begins;  they  seem  to  merge;  a  flame 
seems  to  be  both  light  and  heat.  All  these,  and  many 
similar  phenomena,  indicate  the  close  relationship  of  the 
two  forms  of  energy. 

What  Light  Is.  —  Now  we  come  to  the  question,  what 
is  light  ?  Heat  we  found  to  be  molecular  motion,  the  heat 
that  we  feel  being  the  effect  that  this  motion  produces 
on  us.  But  did  it  occur  to  you  to  ask,  when  studying  heat, 
how,  if  it  be  molecular  motion,  does  it  get  to  us  from  the 
sun  through  the  tremendous  spaces  which  are  practically 
empty,  practically  without  molecules?  Now  you  are  in 
a  position  to  understand  this. 

Heat,  we  can  still  say,  is  molecular  movement  wherever 
there  are  molecules  to  move,  and,  since  our  personal  experi- 
ences are  confined  to  spaces  which  are  well  filled  with  mole- 
cules, this,  for  practical  purposes,  does  pretty  well  as  a 
definition  of  heat.  But  we  must  also  allow  that  heat 
passes  through  space  which  is  empty  of  molecules.  This 
we  explain  on  the  theory  that  heat  is  vibration,  and  that 
its  passage  through  empty  space  is  as  vibrations  of  the 
ether.  What  ether  is  no  one  knows,  but  it  is  the  physicists' 
term  for  whatever  it  is  that  occupies  empty  (molecularly 
empty)  space.  When  these  vibrations  of  the  ether  en- 
counter the  fringes  of  the  atmosphere,  they  cause  vibra- 
tion of  the  atmospheric  molecules.  So  we  may  properly 
say  that,  from  here  on,  heat  is  molecular  motion.  These 
vibrations  or  motions  which  constitute  heat  are  wave-like, 
and  these  waves,  by  means  of  fine  instruments,  are  actually 
measurable  as  to  their  length  and  as  to  the  speed  with 
which  they  travel. 

Have  you  ever  tossed  a  stone  into  a  pool  of  still  water, 


218  ELEMENTARY  SCIENCE 

and  watched  the  ripples  as  they  circle  outward,  getting 
fainter  and  fainter  until  they  are  imperceptible?  Here  is 
another  case  of  the  vibrations  by  which  radiant  energy  is 
transmitted,  and,  since  it  is  a  case  in  which  waves  may 
actually  be  seen,  it  may  help  us  understand  the  cases  in 
which  the  waves  cannot  be  seen.  We  see  waves  only  on 
the  surface  of  water,  but  invisible  ones  also  travel  down- 
ward through  it. 

We  must  not  think  of  a  wave  as  the  motion  of  a  mass;  it 
is  rather  the  motion  of  a  motion  as  it  travels  through  a  mass, 
whether  this  mass  be  air,  or  water,  or  a  sheet  that  you 
wave  in  your  arms.  So  a  wave  on  water  is  not  a  mass  of 
water  which  moves  forward;  it  is  rather  the  effect  of  a 
vibration  of  water  which  produces  undulations  as  it  pro- 
gresses. 

Have  you  ever  had  your  head  under  water  when  some 
one  struck  stones  together  near  you  ?  Then  you  know  that 
sound-waves  travelling  through  water  produce  on  us  an 
altogether  different  effect  from  that  produced  when  they 
travel  through  air. 

Now  you  can  better  appreciate  the  kinship  between  light 
and  heat.  For  light  and  heat  (and  sound,  too)  are  all 
simply  vibrations  of  wave-like  nature,  which,  considered 
from  the  standpoint  of  physics  (that  is,  apart  from  the 
effects  they  produce  on  living  things),  are  entirely  of  the 
same  nature  except  for  the  difference  in  their  wave-lengths. 
So  you  see  that  all  forms  of  radiant  energy  are  alike  as 
to  their  essential  nature;  they  differ  only  with  respect 
to  wave-length.  They  also  differ  with  respect  to  the  speed 
at  which  they  travel  and  with  respect  to  the  effects  they 
produce,  but  both  of  these  things  appear  to  be  merely 
results  of  differences  in  wave-length. 


LIGHT,  COLOR,  AND  SOUND  219 

Now  you  can  understand  that  the  apparent  merging  of 
light  and  heat  is  more  than  an  appearance;  it  is  an  actual 
fact,  for  there  are  waves  of  radiant  energy  of  certain  lengths 
that  do  actually  produce  upon  our  sense-organs  the  effects 
of  both  light  and  heat.  Also  you  can  see  that,  by  inter- 
fering with  the  waves  of  light  or  heat  rays,  and  thereby 
altering  their  length,  heat  may  be  changed  to  light,  or 
light  to  heat.  Similarly,  as  you  have  doubtless  noted, 
the  little  waves  on  water  change  their  length  when  they 
strike  a  rock  or  are  otherwise  interfered  with. 

So  we  may  go  back  to  our  question,  what  is  light?  We 
find  that  it  really  isn't  a  thing,  but  it  is  an  effect.  That 
is,  it  isn't  a  thing  any  more  than  pain  or  pleasure  are  real 
things.  It  is  simply  an  effect  produced  on  us,  through  the 
instrumentality  of  our  eyes,  by  waves  of  radiant  energy 
of  certain  lengths.  Eyes  different  from  ours  may  get 
entirely  different  sensations  of  light  from  ours.  Thus 
we  say  that  a  cat  can  see  at  night,  while  to  an  insect  this 
undoubtedly  appears  to  be  a  very  different  world  from 
what  it  appears  to  us.  You  know  that  a  tuning-fork 
across  the  room  will  respond  to  sound  vibrations  of  a  cer- 
tain pitch  which  are  set  up  by  striking  a  note  on  the  piano. 
Similarly,  we  may  regard  some  of  our  sense-organs  as  in- 
struments tuned  up  to  certain  pitches;  to  certain  kinds  of 
vibrations  they  respond,  to  others  they  do  not.  If  there 
were  no  eyes  to  see,  or  ears  to  hear,  or  skin  to  feel,  would 
the  things  we  call  light,  sound,  and  heat  then  really  exist? 

Light-waves  are  shorter  than  those  of  heat,  and  travel 
faster.  They  are  very  much  shorter  than  those  of  sound, 
and  travel  correspondingly  faster.  This  fact  you  have 
witnessed  many  times.  You  have  seen  the  steam  rise 
from  a  locomotive-whistle  before  you  have  heard  the 


220  ELEMENTARY  SCIENCE 

whistle;  you  have  seen  the  flash  of  a  gun  before  you  heard 
the  report;  and  you  have  seen  the  lightning  flash  before 
you  have  heard  the  thunder.  Sound  travels  at  the  rate 
of  about  eleven  hundred  feet  a  second,  but  light  has  the 
tremendous  velocity  of  one  hundred  and  eighty-six  thousand 
miles  a  second.  Yet,  as  you  may  have  heard,  so  remote 
are  some  of  the  distant  stars  that  it  requires  many  years 
for  their  light  to  reach  us. 

Color.  —  Now  light-waves  themselves  are  not  all  of  the 
same  length.  What  we  see  as  white  light  is  simply  the 
composite  effect  of  light-rays  whose  wave-lengths  are 
slightly  different.  It  is  possible  to  separate  the  rays 
which  compose  white  light  in  such  a  manner  that  each 
kind  of  ray  produces  a  separate  effect.  You  have  seen 
this  result,  and  you  have  often  marvelled  at  the  beauty 
of  it.  You  have  seen  it  whenever  you  have  looked  at  a 
rainbow. 

A  rainbow  gives  us  what  is  called  the  solar  spectrum, 
by  which  is  meant  a  separation  of  the  visible  rays  of  the 
sun  in  such  a  manner  that  each  kind  produces  its  own  effect 
upon  the  eye.  These  different  effects  we  call  colors. 
As  you  may  have  learned  in  using  paint  or  water-colors, 
white  is  not  a  color,  but  is  a  "combination  of  all  colors," 
while  black  is  "the  absence  of  color." 

Now  we  are  in  a  position  to  define  color.  Why  is  it 
that  one  object  looks  green  while  another  may  look  red? 
It  is  simply  because  the  surface  of  the  green  object  is  of 
such  nature  that  it  absorbs  all  the  rays  of  light  that  strike 
it,  except  those  which  produce  the  effect  of  green.  Similarly, 
a  red  object  reflects  only  those  which  produce  an  effect 
of  red.  So  we  may  define  the  color  of  an  object  as  the 
effect  produced  on  us  by  the  light-rays  which  it  reflects.  For 


LIGHT,  COLOR,  AND  SOUND  221 

example,  leaves  look  green  to  us  not  exactly  because  they 
are  green  (though  it  is  quite  right  to  say  they  are),  but 
because  the  light  which  they  fail  to  absorb  and  which 
they  reflect  back  to  our  eyes  produces  an  effect  of  green. 

Surely  you  have  noticed  that  the  color  of  objects  is  not 
quite  the  same  when  viewed  by  artificial  light  as  when 
viewed  by  daylight;  thus,  by  artificial  light  it  is  hard  to 
tell  blue  from  green.  Evidently,  then,  the  color  of  an 
object  depends  to  some  extent  upon  the  nature  of  the  light 
that  is  shining  upon  it. 

This  suggests  an  interesting  and  significant  fact.  If 
gaslight,  for  example,  produces  light  effects  that  are  differ- 
ent from  those  produced  by  sunlight,  does  it  not  follow 
that  the  spectrum  of  gaslight  will  be  different  from  the 
spectrum  of  sunlight?  If  an  object  which  is  blue  in  sun- 
light is  not  blue  in  another  kind  of  light,  does  it  not  follow 
that  the  other  kind  of  light  lacks  blue  rays?  For  surely 
the  object  does  not  change  its  nature  whenever  it  is  changed 
from  one  kind  of  light  into  another;  the  change  of  color 
must  be  due  to  differences  in  the  kinds  of  light. 

Such  thoughts  as  these  led  scientists  to  examine  care- 
fully the  spectra  produced  by  various  kinds  of  light,  and 
it  was  found  to  be  a  fact  that  every  kind  of  substance  capa- 
ble of  releasing  light  releases  a  kind  which  is  di/erent  from 
all  other  kinds.  Thus  certain  substances  when  they  burn 
release  light  which  contains  no  blue  rays.  Thus  we  see 
why  an  object  blue  by  daylight  may  fail  to  be  blue  by 
artificial  light. 

Now  since  it  has  been  found  that  every  kind  of  substance 
when  it  burns  releases  its  own  kind  of  light,  it  follows  that 
if  the  spectrum  characteristic  of  a  certain  substance  be 
known,  then  it  may  be  possible  to  tell  what  a  substance 


222  ELEMENTARY  SCIENCE 

is  by  merely  obtaining  the  spectrum  of  its  light.  It  is  by 
this  means  that  scientists  have  been  able  to  discover 
important  facts  about  the  chemical  nature  of  heavenly 
bodies.  So  far  as  their  researches  have  gone,  they  have 
been  unable  to  discover  any  element  in  the  heavenly  bodies 
which  is  different  from  the  elements  here  on  earth.  Does  not 
this  suggest  a  common  origin  for  all  the  heavenly  bodies? 

Reflection  of  Light.  —  We  have  noted  that  light  may  be 
absorbed  or  may  be  reflected.  Light-rays,  unless  inter- 
fered with,  travel  in  straight  lines, 
and  the  principle  of  their  reflection 
is  the  same  as  the  principle  which 
governs  the  rebound  of  a  ball. 
That  is,  the  angle  of  incidence  is 
equal  to  tlie  angle  of  reflection  (see 
Fig.  80).  We'  see  this  principle  in 
operation  when  a  billiard-ball  re- 
bounds from  a  cushion,  or  a  tennis- 
ball  from  a  concrete  sidewalk,  or  we  may  study  it  very  in- 
terestingly by  observing  the  way  in  which  mirrors  reflect 
images. 

Refraction.  —  There  are  other  things  than  absorption 
and  reflection  which  may  happen  to  light-rays  when  they 
encounter  surfaces.  One  of  these  is  refraction,  which  means 
that  the  light-ray  is  bent.  This  explains  why  a  spoon, 
or  an  oar,  or  anything  else,  looks  bent  when  partly  immersed 
hi  water.  It  seems  to  bend  at  the  point  where  it  enters 
the  water.  This  is  because  of  the  greater  density  of  water 
as  compared  with  air;  because  of  this  difference  in  density, 
light  passes  through  water  in  a  direction  somewhat  altered 


LIGHT,  COLOR,  AND  SOUND 


223 


from  that  of  its  passage  through  air;  rays  of  light,  as  they 
pass  from  a  less  dense  to  a  more  dense  medium,  or  the 
reverse,  are  modified  in  direction.  So  the  light  reflected 
back  to  our  eye  from  the  part  of  an  object  that  is  under 
water,  changes  slightly  in 
direction  as  it  passes  from 
the  surface  of  the  water 
(see  Fig.  81).  But  our 
eyes  see  things  only  in 


Straight  lines;  they  do  FlG-  8 T-~ Diagram  showing  how  the  light- 
waves f:om  object  E,  under  water,  are 
bent  (refracted)  as  they  go  from  water 
into  air  at  various  angles. 


not  make  the  correction 

which  refraction  requires 

in  order  to  give  us  the  right  position  of  an  object.    We 

must  make  this  correction  with  our  brains,  as  boys  do 

when  shooting  or  spearing  for  fish. 

Diffusion.  —  We  should  also  note  that  light  upon  en- 
countering rough  surfaces  is  dispersed  in  all  directions  rather 
than  being  reflected  in  straight  lines  (see  Fig.  82).  Thus 
ground  (roughened)  glass  reflects  light  irregularly  rather 


FIG.  82. — D:agram  showing  how  smooth  and  rough  surfaces  reflect  light. — After 
MILLIKAN  and  GALE. 

than  in  straight  lines,  or  permitting  it  to  pass  through. 
Such  dispersal  or  uneven  reflection  of  light  is  called  di/usion. 
We  have  noted  the  great  importance  of  the  diffusion  of 
light  by  dust  and  water  particles  in  the  atmosphere.  It 
is  this  diffused  light  which  lights  up  the  shaded  places. 


224  ELEMENTARY  SCIENCE 

Except  for  it,  there  would  be  stronger  contrasts  between 
sunlight  and  shadow.  Such  contrasts  are  characteristic 
of  dry,  clear  air,  as  in  Arizona  or  New  Mexico. 

Transmission  of  Light.  —  If  the  light  which  encounters 
a  substance  simply  continues  on  its  way  without  modifica- 
tion we  say  that  it  is  transmitted,  and  we  call  the  substance 
through  which  it  is  transmitted  transparent  or  translucent, 
depending  upon  the  extent  to  which  light  passes  through 
it;  if  it  does  not  transmit  light,  we  call  it  opaque.  Smooth 
glass  is  transparent,  while  a  sheet  of  white  paper  is  trans- 
lucent. A  substance  may  both  permit  transmission  and 
cause  reflection.  This  you  have  noted  in  passing  show- 
windows  which  are  in  the  shade;  they  reflect  your  image 
while  at  the  same  time  permitting  you  to  see  through 
them. 

So  we  may  sum  up  by  saying  that  the  various  things 
that  happen  to  light  are  transmission,  reflection,  absorption, 
diffusion,  and  refraction.  You  will  find  that,  in  consequence 
of  these  various  light  phenomena,  various  effects  are  pro- 
duced which  are  of  great  importance  to  all  of  us. 

QUESTIONS 

1.  Why  are  black  clothes  warmer  than  white? 

2.  Why  are  greenhouses  warmer  than  the  air  outside? 

3.  What  is  the  difference  between  light  and  heat? 

4.  How  can  light  be  changed  to  heat? 

5.  How  do  the  waves  of  light,  heat,  and  sound  compare  in  length 

and  speed  ? 

6.  What  is  a  rainbow? 

7.  Explain  how  color  is  produced. 

8.  Why  does  a  stick  look  bent  when  partly  immersed  in  water? 

9.  What  is  the  difference  between  transparent  and  translucent 

objects? 


CHAPTER  XXIX 

SOME  EFFECTS  OF  LIGHT.    PHOTOGRAPHY 
PHOTOSYNTHESIS 

Cause  of  the  Spectrum.  —  You  have  learned  that  the 
rainbow  is  due  to  the  separation  of  sunlight  into  its  rays 
of  different  wave-lengths.  You  have  also  noticed  that 
rainbows  appear  only  when  the  sun  is  low  enough  in 
the  sky  that  the  effects  produced  by  fine  drops  of  water 
are  reflected  back  to  us  from  the  other  side  of  the  heavens. 
Now  that  you  have  studied  refraction,  you  can  understand 
why  drops  of  water  may  cause  the  breaking  up  of  light 
into  its  spectrum.  Each  kind  of  ray  is  refracted  in  a  slightly 
different  manner  from  the  other  kinds,  dependent  upon  the 
nature  of  its  wave-length.  Then,  if  there  be  behind  the 
refracting  surface  another  surface  that  catches  and  reflects 
back  to  us  the  rays  that  have  thus  become  separated  into 
their  own  kinds,  our  eyes  receive  the  image  of  a  spectrum. 

You  have  seen  rainbows  in  the  spray  of  a  fountain  or 
lawn-sprinkler.  The  drops  of  water,  just  as  in  the  case  of 
a  rainbow  in  the  sky,  act  both  as  refractors  and  as  reflectors. 

Also  you  can  see  why  a  glass  prism  may  produce  a 
spectrum.  Note  that  in  the  spectrum  produced  by  a 
prism  in  the  manner  shown  in  the  figure  the  colors  occur 
in  the  following  order,  reading  upward:  red,  orange, 
yellow,  green,  blue,  indigo,  and  violet.  This  is  because 
the  red  rays,  whose  wave-lengths  are  longest,  are  refracted 
least,  while  the  violet  rays,  whose  wave-lengths  are  shortest, 
are  refracted  most.  This  order  of  the  colors  of  the  spec- 
335 


FIG.  83. — Diagram  showing  the  effect  of  the  convex 
lens  on  light-waves. 


226  ELEMENTARY  SCIENCE 

trum  is  useful  to  remember;  you  can  do  so  easily  by  re- 
calling the  initials  of  the  colors  in  the  proper  order,  namely, 
roygbiv. 

Lenses  are  bits  of  glass  very  carefully  manufactured  so 
as  to  be  perfectly  uniform  in  texture  and  then  very  care- 
fully shaped  by 
grinding  them  so 
that  their  refrac- 
tion of  light  is 
geometrically  per- 
fect. A  convex 

lens  (see  Fig.  83)  refracts  light  so  that  it  converges  to  a 
point  (the  focus)  at  the  side  of  the  lens  opposite  to  the 
source  of  light.    A  concave  lens  refracts  light  so  that  it  does 
not  converge,  but  spreads  (see  Fig.  84).    Now  suppose 
that  your  eye  is  at  the  point  marked  F  in  Fig.  83,  and  that 
you  are  trying  to  look  through  the  lens  at  an  object  on  the 
other  side  of  it.    First  you  will  so  adjust 
the  distance  between  your  eye  and  the 
lens  that  the  image  of  the  object  comes  to 
a  focus  upon  that  part  of  your  eye  (the 
retina)  that  is  sensitive  to  images,  for 
otherwise  the  image  will  appear  blurred 
or  you  cannot  see  it  at  all.    Do  you  not 
see  that  the  effect  produced  by  thus  look- 
ing at  an  object  through  a  convex  lens  is  to  make  it  look 
larger;  that  is,  to  magnify  it?    If  you  remove  the  lens  and 
look  at  the  object  with  the  naked  eye,  it  at  once  looks  smaller. 
A  concave  lens  has  the  reverse  effect.    Thus  you  can  see 
why  convex  lenses  are  used  in  the  glasses  for  near-sighted 
eyes,  while  concave  lenses  are  used  for  far-sighted  eyes. 


FIG.  84.  —  Diagram 
showing  the  effect  of 
the  concave  lens  on 
light-waves. 


SOME  EFFECTS  OF  LIGHT 


227 


A  microscope  (mikros,  small;  scopein,  to  appear)  is  an 
instrument  to  make  "small  objects  appear,"  as  the  origin 
of  its  name  very  nicely  indicates.  That  is,  it  permits  us, 
by  enlarging  their  images,  to  see 
objects  which  otherwise,  on  ac- 
count of  their  smallness,  we 
could  not  see.  A  compound 
microscope  is  one  in  which  sev- 
eral  lenses  are  used,  and  in 
which  the  object  is  examined  by 
the  aid  of  light  which  is  reflected 
through  it  from  below  upward 
(study  Fig.  85). 


FIG.  85. — Diagram  showing  the 
principle  of  the  compound  mi- 
croscope. 


A  telescope  (tela,  far)  is  an  in- 
strument which  enlarges  the  im- 
age of  remote  objects  and  so 
seems  to  bring  them  near.  You 
can  readily  see  that  the  perfection  of  both  these  instruments 
has  depended  upon  the  perfection  of  the  art  of  lens  manu- 
facture. 

Photography  (photos,  light;  graphein,  to  write),  or  the 
"writing  of  light,"  depends  upon  various  effects  produced 
upon  light  and  by  it.  It  depends  upon  the  refraction  of 
light  by  the  lens  of  the  camera,  so 
that  the  light  reflected  from  the 
objects  to  be  photographed  is 
focussed  upon  the  sensitive  plate  or 
film  upon  which  the  image  or  "pic- 

FIG.  86.— Diagram  illustrating       ture"     is    to    be    recorded.       Since 
tte  principle  of  photograph-      Ught  f  ^  Qther  ^^  WQuld  c(m_ 


228  ELEMENTARY  SCIENCE 

fuse  the  result  (spoil  the  picture),  the  construction  of  the 
camera  must  be  such  that  light  enters  it  only  from  the 
objects  which  are  to  be  photographed  (study  Fig.  86). 
The  recording  of  the  image  depends  upon  the  fact  that 
light  is  absorbed  by  the  substance  (usually  silver  chloride 
or  bromide)  which  coats  the  plate  or  film,  and  upon  being 
absorbed  it  decomposes  this  substance.  Thus  the  image  of 
white  objects  appears  black  when  the  plate  or  film  is  de- 
veloped, dark  objects  appear  light,  and  gray  effects  are 
produced  by  objects  which  were  intermediate  between 
light  and  dark  (colored).  You  can  readily  understand  that 
this  is  because  the  light  reflected  from  light  objects  de- 
stroys the  chemical  coating,  while  dark  objects  can  produce 
no  such  effect  because  they  do  not  reflect  light.  Also  you 
can  see  why  the  developed  plates  or  films  are  called  nega- 
tives. Positives,  in  which  the  normal  appearance  is  re- 
stored, are  produced  by  placing  the  negatives  over  specially 
prepared  paper,  and  exposing  both  to  sunlight.  The 
paper  will  then  be  altered  as  the  negative  was,  but  with 
reverse  effects.  This  process  is  called  printing  the  picture. 

Candle-Power.  —  The  intensity  or  strength  of  light  is 
usually  measured  in  terms  of  a  unit  called  candle-power. 
As  a  basis  for  this  system  of  measuring  there  must  be,  of 
course,  a  standard  type  of  candle,  uniform  in  all  its  details. 
The  candle-power  of  an  electric  lamp  may  be  measured 
as  indicated  in  Fig.  87.  Suppose  the  candle  be  set  just 
one  foot  from  the  upright  rod.  Then  we  will  move  the 
lamp  along  line  ab  until  we  find  the  point  at  which  the 
shadow  it  casts  on  the  screen  is  just  equal  in  intensity 
(darkness)  with  the  shadow  cast  by  the  candle.  Suppose 
this  point  is  just  four  feet  from  the  rod.  We  know  then 


SOME  EFFECTS  OF  LIGHT 


229 


that  the  lamp  has  sixteen  candle-power,  for  light  decreases 
in  density  as  the  square  of  its  distance  from  the  source, 
That  is,  if  you  are  twice  as  far  from  a  lamp  as  some  one 
else,  you  are  getting  (at  least  directly  from  the  lamp) 
only  one-fourth  as  much  light,  which  is  easy  to  understand 
when  you  remember  that  the  light  is  radiating  out  in  every 
direction.  So  we  see  that  an  electric  lamp  which,  at  four 


FIG.  87. — Diagram  illustrating  how  candle-power  may  be  measured,  as  explained 
in  text. 

feet,  produces  the  same  intensity  of  light  or  shadow  as  a 
candle  at  one  foot,  has  a  candle-power  of  four  squared, 
that  is,  sixteen. 

Photosynthesis.  —  You  have  noted  the  great  importance 
to  man  of  certain  effects  that  light  produces  on  him.  Yet 
far  more  important  to  man  than  any  of  these  is  an  effect 
which  it  does  not  produce  on  him;  an  effect  which  it  pro- 
duces directly  upon  green  plants,  but  which,  indirectly, 
is  of  vast  importance  to  all  other  living  creatures. 

Blindness  is  one  of  the  greatest  of  misfortunes,  and  yet 
one  can  live  without  seeing;  that  is,  he  can  live  without 
witnessing  all  the  effects  of  light  which  we  have  been  con- 


230  ELEMENTARY  SCIENCE 

sidering,  which,  to  perceive,  one  must  have  the  power  of 
sight.  But  even  blind  people  could  not  live  were  it  not 
for  the  effect  of  light  upon  green  leaves.  For  every  man 
must  have  food,  and  the  basis  of  all  food-supply  is  that 
manufacture  of  it  which  goes  on  in  green  leaves  when, 
and  only  when,  light  is  received  by  them. 

This  process  of  food  manufacture  in  green  leaves  you 
have  already  learned.  It  is  called  photosynthesis,  which, 
except  for  its  length,  is  a  very  good  name  for  it.  It  is  a 
.word  which  explains  its  own  meaning.  The  photo  in  it 
means  light.  Syn  means  together.  Thesis  means  putting. 
So  the  whole  word  may  be  translated  putting  together  in 
the  light,  and  it  refers  to  the  power  of  the  plant  to  put  to- 
gether in  the  light  certain  substances  in  such  a  way  that  they 
form  food,  food  of  the  class  known  as  carbohydrates,  of  which 
starch  and  sugar  are  examples.  Photosynthesis  is  not 
all  that  there  is  to  food  making.  The  food  which  is  made 
by  photosynthesis  may  be  afterward  transformed  into 
other  kinds  of  foods.  But  photosynthesis  is  the  only 
process  by  which  food  is  made  out  of  materials  which  are 
not  themselves  food.  We  ourselves  have  the  power  to 
transform  foods  from  one  kind  into  another,  but  we  do 
not  have  the  power  to  make  food  out  of  materials  which 
are  not  food.  This  is  done  only  by  green  plants.  In  the 
foods  which  they  manufacture  they  store  up  energy  which 
is  derived  from  sunlight. 


SOME  EFFECTS  OF  LIGHT  231 

QUESTIONS 

1.  Explain  the  rainbow. 

2.  What  is  a  spectrum,  and  how  is  it  produced? 

3.  How  do  convex  and  concave  lenses  affect  light-rays? 

4.  What  is  the  principle  of  a  compound  microscope  and  of  a 

telescope  ? 

5.  How  are  photographs  made? 

6.  What  relation  does  light  hold  to  food  production? 

7.  Why  are  leaves  green? 


CHAPTER  XXX 
FOOD.    THE  NUTRITIVE  CYCLE 

You  have  just  noted  that  photosynthesis  is  the  manufac- 
ture of  only  one  kind  of  food  —  carbohydrate  food.  Evi- 
dently, it  is  important  for  us  to  find  out  what  other  kinds 
of  food  there  are.  We  have  been  studying  inorganic  phe- 
nomena of  nature,  especially  with  reference  to  their  rela- 
tions to  life.  Now  if  we  are  to  see  how  this  inorganic 
realm  of  nature  is  related  to  life  in  the  most  important 
way  of  all,  we  must  surely  study  food.  For  it  is  by  means 
of  food  that  the  actual  stuff  of  life  (protoplasm}  and  the 
structures  that  it  inhabits  (bodies]  are  built  up  out  of  the 
inorganic  materials  of  earth. 

What  Food  Is.  —  So  let  us  first  ask  ourselves,  what  is 
food?  It  is  a  popular  rather  than  a  scientific  term,  and 
does  not  have  a  very  precise  meaning.  Ordinarily  it 
refers  to  anything  which,  when  taken  into  the  body,  con- 
tributes to  the  growth,  energy,  or  repair  of  the  body. 
So  we  may  take  this  as  our  "working  definition"  of  food. 
But  in  applying  this  definition,  some  questions  arise.  Is 
water  a  food?  We  shall  have  to  admit  that  it  contributes 
to  the  growth  of  the  body.  Is  air  a  food  ?  We  shall  have 
to  admit  that  it  is  essential  to  the  release  of  energy  in  the 
body.  And  yet  air  and  water  are  quite  different  from  the 
things  we  ordinarily  think  of  as  food  —  such  things  as 
bread,  meat,  and  potatoes. 

232 


FOOD 


233 


Bread,  meat,  and  potatoes  are  organic  substances,  while 
air,  water,  and  salt  are  inorganic  substances.  So,  if  we  are 
to  regard  the  latter  group  as  food,  we  evidently  should 
recognize  a  great  and  fundamental  distinction  that  exists 
between  organic  and  inorganic  foods;  it  is  only  the  organic 
foods  that  can  supply  energy  to  living  things  or  contribute 
to  their  living  substance;  "inorganic  foods"  are  necessary 
to  life,  and  yet  they  have  an  entirely  different  relationship 
to  it  from  that  of  the  organic  foods;  they  provide  the 
physical  conditions  under  which  life  operates  rather  than 
providing  anything  which  becomes  a  part  of  life  itself. 

To  some  writers  this  distinction  seems  so  important  that 
they  use  the  word  food  as  applied  to  organic  substances  only. 
Such  usage  makes  the  elementary  study  of  food  much 
clearer.  Thus  when  you  consider  photosynthesis,  you  find 
that  in  this  process  green  plants  transform  certain  inorganic 
substances  (water  and  carbon  dioxide)  into  a  certain  class 
of  organic  substances  (carbohydrates).  Now  shall  we  call 
both  the  product  of  this  process  and  the  raw  materials  it 
uses  by  the  same  name,  i.  e.,  food?  If  we  do  so,  we  shall 
be  saying  that  plants  manufacture  food,  and  at  the  same 
time  be  saying  that  they  get  their  food  from  the  air  (car- 
bon dioxide)  and  from  the  soil  (water) .  This  is  so  confusing 
and  inconsistent  that  it  is  evidently  better  to  call  only 
the  finished  product  food,  and  to  call  the  raw  materials 
food-substance  or  food-material.  Thus  we  not  only  make 
our  meaning  more  clear,  but  we  emphasize  one  of  the  two 
most  fundamental  changes  which  occur  in  what  we  call 
the  cycle  of  life,  namely,  the  transformation  of  inorganic 
substances  into  organic  ones.  The  other  great  change  is 
the  transformation  of  organic  substances  back  to  inorganic, 
thus  completing  the  cycle. 


234  ELEMENTARY  SCIENCE 

Carbohydrates.  —  You  have  learned  that  photosynthesis 
produces  carbohydrates,  of  which  starch  and  sugar  are 
examples.  Carbohydrates  are  substances  composed  of 
carbon,  hydrogen,  and  oxygen  only,  and  in  their  molecules 
the  atoms  of  hydrogen  and  oxygen  always  occur  in  the 
proportion  of  two  to  one,  as  in  water.  Thus  the  molecule 
of  a  common  kind  of  sugar  (there  are  many  kinds)  has  the 
formula  CeHi^e,  which  indicates  that  a  molecule  of  this 
kind  of  sugar  is  composed  of  six  atoms  of  carbon,  twelve  of 
hydrogen,  and  six  of  oxygen.  The  proportion  of  carbon  in 
different  carbohydrates  varies,  but  that  of  hydrogen  and 
oxygen  always  remains  the  same.  Now  in  photosynthesis, 
for  every  molecule  of  water  (H2O)  used  it  is  believed  that 
a  molecule  of  carbon  dioxide  (C02)  is  also  used.  Since  the 
proportion  of  hydrogen  to  oxygen  in  these  two  kinds  of 
molecules  taken  together  is  two  of  hydrogen  to  three  of 
oxygen,  it  is  evident  that  in  the  manufacture  of  carbohy- 
drates from  these  substances  (photosynthesis),  there  will  be 
a  considerable  surplus  of  oxygen.  This  you  have  already 
learned  in  considering  the  maintenance  of  the  oxygen- 
supply  in  the  atmosphere. 

Most  of  our  food  contains  more  or  less  of  carbohydrate. 
Carbohydrate  foods  are  of  especial  importance  as  sources 
of  the  energy  that  we  expend. 

Fats.  —  The  fats  constitute  another  great  class  of  foods. 
You  have  already  learned  in  connection  with  your  study 
of  combustion  that  fat  is  composed  of  just  the  same  ele- 
ments as  carbohydrate.  The  chief  difference  is  that  the 
proportion  of  oxygen  in  them  is  a  great  deal  less  than 
to  is  in  carbohydrates.  So  fats  have  a  greater  oxygen- 
hunger  than  have  the  other  kinds  of  foods;  they  burn 


FOOD  23S 

much  more  readily.  Have  you  not  seen  how  easily 
the  fat  catches  fire  when  steak  is  broiled  ?  Fats  are  im- 
portant in  our  diet  especially  for  their  fuel  value;  they 
help  to  keep  us  warm;  we  eat  more  of  them  in  winter  than 
in  summer. 

Proteins.  —  The  third  great  class  of  foods  is  the  class 
called  proteins,  and  now  we  have  reached  a  subject  concern- 
ing which  our  ignorance  is  greater  than  our  knowledge. 
For  the  science  of  chemistry  has  not  yet  revealed  the  exact 
composition  of  the  molecules  of  proteins;  we  cannot  write 
a  chemical  formula  for  them  as  we  can  for  fats  and  carbo- 
hydrates. Why  not?  Simply  because  these  substances 
are  so  complex  that  they  defy  chemical  analysis.  As 
soon  as  the  chemist  begins  to  work  with  them,  their  mole- 
cules "break  down"  into  other  substances.  Then  all  he 
can  do  is  to  determine  what  are  the  elements  which  com- 
pose their  molecules.  As  to  the  proportions  of  these  ele- 
ments in  the  protein  molecule,  or  as  to  how  they  are  ar- 
ranged with  reference  to  one  another,  he  is  quite  at  a  loss 
to  tell.  So  you  see  why  we  have  no  definite  formulas  for 
protein  molecules;  our  knowledge  of  them  is  quite  limited. 

Yet  we  do  know  that  proteins  contain  all  the  elements 
that  carbohydrates  and  fats  contain,  and  a  number  of 
others  besides.  Of  these  others,  nitrogen  is  the  most 
conspicuous.  Indeed,  it  is  so  conspicuous  that  it  is  cus- 
tomary to  speak  of  the  proteins  as  nitrogenous  foods,  and 
of  all  others  as  non-nitrogenous  foods.  Other  elements 
present  in  proteins  in  minor  and  varying  amounts  are 
phosphorus,  iron,  and  sulphur. 

Proteins  are  essential  for  the  growth  and  repair  of  the 
body.  They  are  the  great  tissue-formers.  Our  muscles 


236  ELEMENTARY  SCIENCE 

are  chiefly  composed  of  them,  and  the  muscles  of  animals 
(meat)  furnish  us  our  chief  supply  of  them.  They  are  also 
present  in  vegetable  foods,  especially  in  peas  and  beans. 
(Why  especially  in  peas  and  beans?) 

Protoplasm.  —  The  proteins  gradually  merge  into  that 
mysterious,  semifluid  substance  which  Huxley  first  called 
the  physical  basis  of  life.  We  call  it  mysterious  because 
we  know  so  little  of  its  origin  or  of  its  nature.  With  its 
behavior  we  are  familiar.  Its  presence  in  all  living  things 
is  observable,  and  all  the  "manifestations  of  life"  seem  to 
depend  upon  its  activity.  It  responds  in  definite  ways  to 
changes  in  the  conditions  which  surround  it,  yet  some  of 
its  activities  do  not  seem  to  be  of  the  nature  of  responses; 
they  seem  to  be  self-initiated;  at  least  scientists  have  never 
been  able  to  determine  the  causes  of  such  activities.  So 
here,  you  see,  we  have  a  substance  which  is  a  sort  of  "holy  of 
holies"  of  life,  into  which  man  with  all  his  cunning  has  not 
yet  been  able  to  penetrate.  In  it  are  the  mysteries  of  life, 
and  whether  it  be  in  the  lowliest  plant  or  in  the  brain  of 
the  wisest  man,  it  has,  for  aught  we  know,  the  same  phys- 
ical nature.  At  least  it  seems  to  be  composed  of  the  same 
elements,  and  no  element  has  ever  been  found  in  it  which 
is  different  from  those  found  in  the  non-living  world. 
When  this  substance  comes  to  be  known,  if  it  ever  is,  the 
secrets  of  life  will  be  known  as  they  have  never  been  known 
before.  This  is  the  substance  which  you  have  already 
learned  to  call  protoplasm. 

The  Nutritive  Cycle.  —  You  should  study  carefully  the 
diagram  on  page  237,  and  try  thoroughly  to  understand  its 
significance.  For  nothing  so  completely  indicates  the 


FOOD 


237 


dependence  of  life  upon  its  environments  as  do  the  facts 
which  this  diagram  represents.  If  you  can  form  a  clear 
picture  in  your  mind  of  what  is  caUed  the  nutritive  cycle 
(which  is  what  this  diagram  portrays),  then  you  have 
grasped  some  of  the  greatest  and  most  fundamental  prin- 
ciples of  life. 

Note  first  the  absolutely  inorganic  nature  of  the  materials 
with  which  the  process  of  food  construction  is  begun;  the 

AIR 


CARBON  DIOXIDE 


ENERGY  FROM 
SUNLIGHT  STORED  UP 


CARBON  DIOXIDE 


ENERGY  SET  FREB 
HEAT 


GREEN  PUNTS 


ANIMALS 

AND  PUNTS 

WITHOUT  CHLOROPHYLL' 


COMPLEX  ORGANIC 
SUBSTANCES  BUILT  UP 

}          I 

COMPLEX  ORGANIC 
SUBSTANCES  BROKEN 
DOWN 

1          \ 

WATER 


SOIL 


NITF 
SALTS  AND 
OTHER  SALTS 


AND  OTHER  SALTS 


FIG.  88.— The  nutritive  cycle. 

materials  which  the  protoplasm  of  green  leaves  uses  in 
the  process,  and  by  means  of  which  this  "living  stuff" 
maintains  and  renews  itself.  These  materials  (carbon 
dioxide  and  water)  existed  on  earth  long  before  life  began, 
and  they  exist  on  other  worlds  than  ours. 

But  let  us  note  at  once  that  more  than  these  raw  ma- 
terials, and  the  protoplasm  which  acts  upon  them,  are 
necessary  to  the  process.  There  must  be  light.  It  is 
from  sunlight  that  green  leaves  absorb  that  radiant  energy 


238  ELEMENTARY  SCIENCE 

that  they  utilize  in  this  process,  and  store  up  in  its  products 
in  latent  form,  to  be  released  again  when  these  products 
are  oxidized. 

The  protoplasm  of  green  leaves  produces  a  substance 
which  acts  as  the  means  whereby  the  needed  sunlight  is 
absorbed.  Evidently,  then,  it  is  this  light-absorbing  sub- 
stance that  makes  the  leaves  look  green.  If  the  spectrum 
of  sunlight  is  examined  after  it  has  passed  through  a  solu- 
tion of  this  substance,  dark  bands  are  found  in  it;  these 
dark  bands  indicate  what  rays  are  absorbed  by  this  sub- 
stance; they  show  us  just  what  are  those  rays  of  sunlight 
upon  which  our  food-supply  depends.  This  light-absorb- 
ing, green-appearing  substance  is  called  chlorophyll.  It 
occurs  in  all  green  parts  of  plants  in  units  of  structure 
called  chloroplasts,  which  are  microscopic  bodies  com- 
posed of  protoplasm  and  the  chlorophyll  which  the  proto- 
plasm secretes.  These  chloroplasts  are,  then,  the  real 
organs  of  photosynthesis,  and  now  you  are  in  a  position  to 
understand  the  scientific  definition  of  this  process,  which 
is  as  follows: 

Photosynthesis  is  the  manufacture  of  carbohydrates  by 
chloroplasts  in  the  presence  and  by  the  aid  of  sunlight,  water 
and  carbon  dioxide  being  used,  and  a  surplus  of  oxygen  being 
given  of. 

Passing  upward  on  the  diagram  beyond  photosynthesis, 
we  note  the  appearance  of  fats  and  proteins,  the  latter 
involving  the  addition  of  a  number  of  new  elements.  Fat 
and  protein  formation  do  not  require  light,  and  may  occur 
in  animals  as  well  as  in  plants,  but  always  carbohydrates 
are  necessary  as  a  basis.  All  these  processes  are,  of  coi 
expressions  of  the  activity  of  the  protoplasm. 

Oxidation  of  foods,  involving  the  release  of  energy 


FOOD 


239 


the  breaking  down  of  their  molecules  into  simpler  com- 
pounds (waste  substances),  may  occur  at  any  point  above 
the  carbohydrate  stage,  as  is  indicated  by  the  arrowed 
lines  that  run  across  the  diagram  from  left  to  right.  Yet 
some  of  the  food,  of  course,  is  destined  to  continue  in  the 
process  of  elaboration  until  it  becomes  a  part  of  the  proto- 
plasm itself.  For  the  protoplasm  itself  is  constantly  break- 
ing down  and  must  constantly  be  renewed.  Now  you  can 
see  why  it  is  believed  to  be  true,  as  you  have  probably 
heard,  that  at  the  end  of  seven  years  every  part  and  par- 
ticle of  the  human  body  has  been  changed,  so  that  nothing 
which  was  there  seven  years  ago  is  there  now;  in  most 
of  the  body  the  changes  or  replacements  are  even  more 
rapid.  Now,  too,  you  can  understand  better  what  was 
meant  by  the  statement  that  life  is  a  procession  of  changes. 
Huxley  said  of  protoplasm  that  it  is  like  a  whirlpool  in  that 
it  is  ever  changing,  yet  still  the  same. 

On  the  descending  side  of  the  circle  you  get  new  evidence 
of  the  immense  importance  of  bacteria.  Here  they  appear 
as  the  chief  assistants  to  the  process  of  oxidation  in  com- 
pleting the  cycle,  in  reducing  dead  and  waste  organic 
stuff  to  the  inorganic  state  again.  What  would  be  the 
disadvantage  to  life  in  case  the  cycle  were  not  complete 
at  this  point? 

The    Artificial    Manufacture    of    Food.  —  Physics   and 

chemistry  are  commonly  considered  to  be  those  branches  of 
science  that  have  contributed  most  to  man's  convenience. 
But  as  you  study  this  diagram  and  consider  its  significance, 
do  you  not  see  that  the  greatest  invention  of  the  future 
will  be  the  invention  of  a  biologist?  Physics  and  chem- 
istry will  continue  to  contribute  much  to  man's  convenience, 


240  ELEMENTARY  SCIENCE 

but  biology  (the  science  of  life)  will  make  the  greatest  con- 
tribution of  the  future  to  his  welfare.  Already  biology  has 
contributed  much  to  the  science  of  health  and  to  the 
science  of  agriculture,  but  the  great  contribution  which  it 
may  make  in  the  future  is  the  artificial  manufacture  of  food. 
For  do  you  not  see  that  if  man  is  once  able  to  understand 
that  process  that  goes  on  in  every  green  leaf,  and  to  du- 
plicate it,  then  the  food  problem  is  forever  solved?  It  is 
simply  a  question  of  taking  certain  simple  and  abundant 
inorganic  materials  and  transforming  them  into  carbo- 
hydrates. It  sounds  simple  enough,  and  it  is  a  thing  which 
occurs  constantly  all  about  us,  wherever  light  and  green 
leaves  come  together,  and  yet  man,  with  all  his  clever  con- 
trivances and  "harnessing"  of  nature,  has  never  yet  been 
able  even  faintly  to  imitate  the  process  of  the  leaf  whereby 
it  "harnesses"  sunlight  and  thereby  works  the  great  trans- 
formation of  inorganic  into  organic. 

Of  course  this  idea  of  the  artificial  manufacture  of  food 
is  purely  speculation,  and  may  never  occur.  Yet  con- 
sideration of  it  should  help  us  to  realize  how  great  is  our 
dependence  upon  plants.  They  are  partners  in  life  with 
us,  and  we,  with  them,  live  only  as  we  are  related  rightly 
to  the  great  inorganic  forces  and  substances  of  the  uni- 
verse, out  of  which  somehow,  mysteriously,  we  have 
come,  in  the  midst  of  which  for  a  very  little  time  we  live, 
and  into  which  our  bodies  will  surely  be  gathered  again 
when  we  go. 


FOOD  241 

QUESTIONS 

1.  What  is  food? 

2.  What  are  carbohydrates  and  how  are  they  produced? 

3.  What  element  is  given  off  when  food  is  manufactured  by  green 

plants,  and  why? 

4.  Which  burn  more  readily,  fats  or  carbohydrates?    Why? 

5.  How  do  proteins  differ  from  fats  and  carbohydrates? 

6.  Why  do  we  call  protoplasm  "the  living  material"? 

7.  What  is  the  process  of  photosynthesis? 

8.  What  is  the  work  of  bacteria  in  the  "cycle  of  life"? 


CHAPTER  XXXI 
PLANT  LIFE 

How  Plants  Work.  —  You  have  seen  how  light  and  heat 
and  air  and  water  and  soil  are  related  to  our  lives.  Now, 
the  green  plants  must  be  added  to  our  mental  picture  be- 
fore we  can  really  understand  how  we  live.  We  must  see 
the  green  plants  at  work  in  the  midst  of  these  other  things 
and  understand  how  they  work. 

Think,  then,  of  a  young  corn-plant.  There  it  stands 
alive  and  at  work  in  the  midst  of  millions  of  others.  Its 
leaves  are  spread  to  the  sunlight  while  its  roots  burrow 
in  the  dark  among  the  soil  grains.  Somehow,  from  soil 
and  air,  it  gathers  materials,  transforms  them  into  food, 
and  stores  this  food  in  the  swelling  grains.  This  is  the 
process  we  must  try  to  understand.  This  is  work  that  all 
green  plants  do.  It  is  work  that  animals  cannot  do;  they 
must  depend  upon  plants  for  this  great  process  of  changing 
inorganic  substances  into  organic  ones,  the  building  up  of 
food.  Plant  bodies,  like  our  own,  cannot  take  in  solid 
materials.  The  substances  that  enter  them  must  be  in 
either  liquid  or  gaseous  form.  So  the  corn-plant  takes  in 
liquids  and  gases.  It  also  gives  out  gas  in  the  form  of 
water  vapor  that  evaporates  from  its  leaves.  It  cannot 
live  without  a  constant  income  and  outgo  of  materials. 
Neither  can  we. 

Roots.  —  The  roots  burrow  deeply  in  the  soil,  and,  near 
their  tips,  they  produce  millions  of  delicate  hairs  called 
242 


PLANT  LIFE 


243 


root-hairs  (see  Fig.  36).  The  root-hairs  have  thin  walls. 
These  thin  walls  are  pressed  closely  to  the  soil  grains,  and 
through  them  the  plant  absorbs  the  liquid  that  it  needs. 
This  liquid  is  the  water  of  the  soil,  and  substances  that  are 
dissolved  in  it.  The  plant  has  no  organs  like  ours  in  which 
the  digestion  of  solids  occurs  before  they  are  absorbed. 
It  has  to  depend  upon  the  water  outside  its  own  body 
to  dissolve  the  substances  that  it  needs  to  take  in. 

The  gas-absorbing  apparatus  is  at  the  opposite  end  of 
the  plant.  Here  the  leaves  stand  in  the  light,  and,  in 
the  skin  of  the  leaves,  usually  on  the  under  surface,  are 
thousands  of  small  pores  through  which  air  passes  freely. 
Up  to  these  leaves  passes  the  water  that  the  roots  take  in, 
and  thence  it  passes  out  into  the  air  as  water  vapor. 

The  absorbing  root-hairs  need  to  be  deep  in  the  soil, 
for  there  is  the  water  that  they  absorb.  Only  deep- 
rooted  corn-plants  can  keep  on  growing  through  a  long 
drought;  the  shallow-rooted  ones  soon  die.  The  leaves, 
on  the  other  hand,  to  do  their  work,  must  be  up  in  the 
air  and  the  light. 

So  we  see  that  there  are  three  great  relations  with  environ- 
ment that  every  food-making  plant  must  have  if  it  is  to  live 
and  grow.  These  three  are  the  water-relation,  the  air- 
relation,  and  the  light-relation.  Of  these  three,  the  air- 
relation  presents  the  simplest  problem,  and  the  water- 
relation  usually  the  most  difficult.  Lack  of  water  kills 
more  plants  than  all  other  causes  combined.  Lack  of 
light  also  kills  many  plants.  In  forests  millions  of  young 
plants  die  because  they  are  overshadowed.  In  dense 
forests  the  light  problem  is  more  difficult  than  the  water 
problem. 

Now  you  can  look  with  the  mind's  eye  at  all  the  green 


244  ELEMENTARY  SCIENCE 

plants  in  the  world,  and  you  can  begin  to  understand  their 
various  forms.  For  you  know  that  all  green  plants  are 
trying  to  do  the  same  thing.  They  are  trying  to  relate 
their  root-hairs  favorably  to  soil-water,  and  their  leaves 
favorably  to  light  and  air.  All  of  the  plant  that  lies  be- 
tween root-hairs  at  one  end  and  leaves  at  the  other  simply 
helps  in  getting  these  two  things  in  good  positions  for 
their  work.  The  deep-burrowing  roots  of  trees  are  neces- 
sary to  anchor  the  trunk  and  branches,  as  well  as  to  reach 
unfailing  supplies  of  water,  and  the  trunk  and  branches 
are  to  get  the  leaves  high  in  the  air  where  they  will  not 
be  overshadowed  by  other  plants. 

Leaves.  —  What  happens  in  the  leaves  ?  Surely  we 
should  understand  this.  Through  all  but  the  winter 
months,  the  leaves  of  plants  cover  all  the  land,  save  where 
man  has  cleared  them  away,  or  where  there  is  not  enough 
water  for  plants  to  grow.  They  are  so  common,  so  beauti- 
ful, and  so  important  to  us  that  every  one  should  under- 
stand their  relation  to  our  own  lives,  and  the  way  in  which 
they  do  their  work.  The  skin  of  a  leaf  may  be  very  harsh 
and  tough,  but  inside  it  is  always  soft  and  tender.  It  is 
filled  with  a  soft  green  pulp.  This  soft  pulp  is  made  up 
of  many  very  small  structures  called  cells.  The  bodies  of 
all  living  things  are  made  up  of  cells  which  exist  in  many 
forms  and  sizes.  In  this  spongy,  soft  interior  of  a  leaf  the 
great  work  of  food-making  goes  on.  Three  things  are 
necessary  to  this  work  of  food-making:  (i)  Sunlight  is 
necessary,  for  it  furnishes  the  energy  used  in  this  work. 
(2)  Air  is  necessary,  for  it  furnishes  carbon  dioxide,  a  sub- 
stance used  in  the  making  of  food.  (3)  Water  is  necessary, 
because  it,  too,  furnishes  materials  that  are  used  in  foods. 


PLANT  LIFE  245 

The  green  parts  of  plants  (whether  leaves  or  stems), 
whenever  the  light  is  shining  on  them,  and  whenever  the 
necessary  materials  are  present,  keep  working  away  at 
their  task  of  food-making.  Food-making  is  an  elaborate 
process.  The  molecules  of  the  substances  out  of  which 
food  is  made  begin  in  the  leaf  a  long  series  of  changes. 
Presently  we  find  them  changed  to  starch  or  sugar,  and 
these  are  true  foods  of  the  carbohydrate  class,  as  you  have 
learned  in  physiology.  It  is  this  work  of  carbohydrate 
manufacture  that  requires  light.  Other  kinds  of  food 
(fats  and  proteins)  may  be  made  out  of  carbohydrates  in 
the  absence  of  light.  To  make  them  out  of  carbohydrates 
does  not  require  the  energy  that  is  necessary  to  make  car- 
bohydrates out  of  carbon  dioxide  and  water. 

Now  you  can  understand  how  a  plant  lives.  You  know 
what  roots,  stems,  and  leaves  are  for.  You  know  that 
they  work  together  in  the  great  process  of  food-making. 
After  the  food  is  made  it  may  pass  in  liquid  form  to  any 
part  of  the  plant.  Thus,  in  the  case  of  corn,  much  of  the 
food  passes  into  the  ear  and  fills  the  grains.  The  most 
important  food  plants  in  the  world  are  the  cereals  —  wheat, 
rice,  corn,  barley,  oats,  etc.  In  all  these  plants  the  food 
used  by  man  is  starch  in  the  seeds.  But  how  do  the  seeds 
help  the  plant  in  its  life?  This  is  another  story,  quite  dif- 
ferent from  the  story  of  root,  stem,  and  leaf. 

QUESTIONS 

1.  Explain  the  work  of  roots. 

2.  What  are  root-hairs,  and  of  what  advantage  are  they  to  the 

plant  ? 

3.  Describe  the  exchange  of  gases  through  the  leaves. 

4.  What  things  are  necessary  for  food-making  by  green  plants? 

5.  What  are  some  of  the  dangers  to  our  crops  ? 


246  ELEMENTARY  SCIENCE 

6.  Why  does  a  tree  have  a  trunk  and  branches?    Do  trunk  and 

branches  work  ? 

7.  What  are  cells? 

8.  Name  some  of  the  most  important  food  plants. 

9.  Does  any  plant  furnish  all  the  kinds  of  food  needed? 


CHAPTER  XXXII 


THE  STORY  OF  SEEDS 

This  is  a  story  quite  worth  telling,  and  a  story  you  can 
easily  understand  if  you  will  try  to  put  yourself  in  the  place 
of  the  plant.  Try  to  see  what  its  problems  are  and  how 
it  solves  them.  The  seed  is  the  plant's 
answer  to  one  of  its  greatest  problems. 

Plants  that  die  each  year  as  winter 
approaches  are  called  annuals;  plants 
that  live  on  from  year  to  year  are  called 
perennials.  You  can  see  that  for  the 
annuals  there  is  the  problem  of  how 
their  young  children  can  live  over  win- 
ter. The  seed  answers  that  problem. 
But  perennials  also  have  seeds.  Let  us 
see  why. 

Our  food  comes  chiefly  from  seeds. 
The  great  crops  of  the  world  are  crops 
of  seeds.  The  food  of  farm  animals  is  largely  composed  of 
seeds.  What  seeds  do  you  know  that  are  used  in  our  foods  ? 
Any  structure  that  contains  seeds  is  a  fruit,  so  the  story  of 
seeds  includes  the  story  of  fruits.  Write  a  list  of  the  different 
kinds  of  fruits  you  have  eaten.  Is  a  tomato  a  fruit  ?  Is  a 
potato  a  fruit? 

We  must  go  back  to  the  time  when  plants  first  began  to 

live  on  the  land.     The  very  first  plants  lived  in  the  water. 

They  did  not  have  roots,  stems,  and  leaves.     They  were 

just  tiny,  round  green  balls,  so  small  that  you  could  not 

247 


FIG.  89.— The  "first" 
plants.— After  J.  M. 
COULTER. 


248  ELEMENTARY  SCIENCE 

see  one  of  them  with  the  naked  eye.  Such  plants  still  live. 
That  green  coating  you  sometimes  see  on  bark,  or  where 
water  drips  on  rock,  is  composed  of  millions  of  such  one- 
celled  plants  (see  Fig.  89). 

Nutrition.  —  Think  of  the  life-problem  of  such  a  plant. 
There  it  floats  near  the  surface  of  the  water  on  a  bright, 
sunshiny  day.  To  keep  alive  it  needs  sunlight  and  air  and 
water  and  some  of  the  substances  dissolved  in  the  water. 
All  these  things  are  abundant  where  it  lives,  and  one  might 
think  that  the  little  plant  would  have  an  easy  time.  This 
is  true  as  long  as  summer  lasts,  but  if  this  little  plant  lives 
where  water  freezes  it  must  change  its  form  to  keep  alive. 
This  it  may  do  by  forming  a  thicker  coat  around  itself, 
and  sinking  to  the  bottom.  There  it  may  rest  for  a  long 
time  and  still  keep  alive  (just  as  seeds  keep  alive)  in  the 
coldest  weather.  When  spring  comes  it  can  come  to  the 
surface  and  start  its  active  life  again. 

Just  keeping  alive  is  not  all  that  there  is  to  plant  life. 
If  it  were,  then  the  plants  on  which  we  live  would  never 
have  come  into  existence,  and  we  should  never  have  come 
into  existence.  All  through  their  history  plants  seem  to 
have  done  everything  in  their  power  to  spread,  to  spread 
everywhere  that  plants  can  live.  There  seems  to  be  no 
place  in  the  world  where  plants  can  grow  but  that,  sooner 
or  later,  they  find  that  place  and  grow  there.  This  means 
that  they  must  reproduce.  It  also  means  that  in  the  long 
history  of  plants  many  different  kinds  have  appeared, 
and  so  we  find  different  kinds  of  plants  in  different  kinds 
of  places.  We  do  not  understand  how  the  different  kinds 
of  plants  have  appeared,  but  we  do  understand  that  they 
all  have  come  from  the  same  ancient  ancestors,  and  that 


THE   STORY  OF  SEEDS  249 

the  plant  kingdom  as  we  see  it  to-day  is  a  result  of  the 
great  tendency  of  plant  life  to  spread  and  multiply. 

To  keep  alive  and  to  multiply,  then,  are  the  two  great 
laws  of  plant  life.  The  former  we  call  nutrition;  the 
latter  reproduction. 

Now,  as  plant  life  went  on  in  those  ancient  days  it  kept 
improving.  The  two  great  problems  of  plant  life  have 
always  been  the  same,  but  the  plant's  ways  of  solving  these 
problems  have  changed.  It  is  as  though  the  plant  king- 
dom kept  inventing  better  ways  of  doing  its  work.  All 
this  took  a  great  deal  of  time.  Plants  are  changing  now 
just  as  they  changed  in  the  past,  but  the  changes  are  so 
gradual  that  we  hardly  notice  them.  Yet  you  know  that 
the  fruits  we  have  in  the  market  to-day  are  much  better 
than  the  fruits  our  grandfathers  had.  This  shows  that 
plants  change,  and  that  these  changes  can  be  controlled 
by  man.  What  we  want  "o  do  now  is  to  consider  some  of 
the  ways  in  which  plants  have  changed  in  the  past,  and 
we  do  this  so  that  we  can  understand  plants  as  we  see  them 
to-day.  • 

Reproduction.  —  There  was  that  ancient  little  plant  that 
had  to  reproduce.  It  did  it  in  the  simplest  possible  way. 
It  just  divided  into  two.  Then  each  of  the  two  new  plants 
(daughter-cells)  divided  again,  and  so  on.  It  was  very 
simple,  and  it  is  a  method  of  reproduction  that  is  still 
used.  But  it  was  not  enough.  Plants  simply  had  to  grow, 
and  expand.  Tiny,  round,  one-celled  plants  could  never 
spread  over  the  land.  They  would  dry  out  too  quickly. 

We  often  find  growing  in  running  water  plants  that  are 
composed  of  slippery  green  threads.  They  belong  to  the 
group  of  plants  called  alga.  Surely  you  have  seen  them 


250 


ELEMENTARY   SCIENCE 


fastened  to  rocks  or  floating  in  the  water  when  you  have 
been  in  wading.  These  threads  are  one  cell  in  thickness, 
but  many  cells  in  length,  as  may  be  seen  under  a  micro- 
scope. In  some  of  these  cells  many  small  bodies  are  found. 
These  little  things  escape  into  the  water  where  they  swim 
around.  They  seem  like  little  animals  they  are  so  active. 
Sometimes  two  of  these  little  things  unite.  After  they  unite, 
they  stop  moving.  They  have  formed  a  single  cell,  and 
this  cell  presently  will  grow  into  a 
row  of  cells  (see  Fig.  90).  This 
production  of  a  new  individual  as  a 
result  of  the  union  of  two  cells  is  the 
sex  process,  or  reproduction,  and 
we  are  going  to  see  what  this  proc- 
ess has  to  do  with  the  production 
of  seeds. 

Now  you  have  seen  two  steps  in 
the  story  of  seeds.  First,  the  little 
plant  that  reproduces  simply  by 
dividing.  Next,  the  little  plant 
that  reproduces  by  the  union  of 
two  cells.  Now  if  these  two  cells 
that  unite  are  of  different  sizes  (as  is  true  of  most  plants), 
we  call  the  smaller  one  the  sperm  and  the  larger  one  the  egg. 
This  is  important  to  remember,  for  all  the  higher  plants 
and  animals  reproduce  by  means  of  sperms  and  eggs. 

If  you  have  ever  been  at  the  seashore  you  have  been 
interested  in  the  tides.  Have  you  ever  noticed  the  plants 
that  lie  on  the  mud  at  low  tide  ?  Probably  the  first  plants 
that  ever  lived  on  land  were  plants  that  were  left  behind 
on  the  mud  by  the  receding  tides.  Of  course  the  greatest 
danger  to  a  plant  is  the  danger  of  drying  up.  All  land- 


FIG.  oo. — One  of  the  algae,  show- 
ing how  the  small  reproduc- 
tive bodies  are  formed  within 
the  cells;  how  they  break 
loose  and  swim  about  (c) ;  how 
they  pair  (d )  and  form  a  single 
cell  (e),  "which  grows  into  a 
new  individual  (D).— After 
J.  M.  COULTER. 


THE   STORY  OF  SEEDS 


251 


plants  are  protected  from  drying  up  by  a  covering  through 
which  water  does  not  easily  evaporate.  So  the  plants 
that  first  lived  on 


FIG.  91. — Pot  of  liverworts,  the  first  land-plants. 


the  land  had  to  de- 
velop a  kind  of  cover- 
ing that  their  water- 
dwelling  ancestors  did 
not  need. 

To-day  we  find 
growing  in  damp 
places  certain  flat 
little  plants  that  look 
as  though  they  had 
rather  recently  come 
out  of  the  water,  and 
are  just  learning  to 
live  on  the  land.  These  plants  are  called  liverworts,  and 
their  favorite  home  is  in  moist  and  shady  places,  especially 
on  damp  cliffs  (see  Fig.  91).  These  plants 
show  us  the  third  step  in  our  story.  They  pro- 
duce eggs  and  sperms,  and  the  sperms  swim 
just  as  they  do  in  algae.  But  the  eggs  stay  where 
they  are  formed.  The  sperms  have  to  come  to 
them.  The  union  of  the  egg  with  the  sperm  is 
called  fertilization.  The  cell  that  results  from 
union  of  egg  and  sperm  is  called  the  fertilized 
egg.  Now  right  here  is  the  new  point  in  the 
story.  The  fertilized  egg  does  not  grow  into  a 
plant  like  its  parent.  It  grows  into  a  club- 
shaped  structure,  and  this  structure  contains 
'p'u'n  t s  °—  hundreds  of  very  small  round  cells.  Presently 
cS5si».M'  the  wall  of  this  structure  bursts  and  the  small 


; 


252 


ELEMENTARY  SCIENCE 


round  cells  escape.  Each  one  of  them,  if  it  reaches  a  favor- 
able place,  will  produce  a  liverwort-plant.  These  little 
round  reproductive  cells  are  called  spores.  Spores  are  repro- 
ductive cells  that  can  grow  directly  into  new  plants.  Eggs 
and  sperms  are  cells  that  cannot  grow  directly  into  new 
plants;  they  must  unite  before  the  new  plant  is  produced. 

Mosses  look  more 
like  regular  land-plants 
than  liverworts  do,  but 
you  nearly  always  find 
mosses  where  you  find 
liverworts,  and  they 
have  the  same  method 
of  reproduction.  They 
belong  to  the  same 
great  group  of  plants 
(see  Fig.  92). 

The  fourth  step  to- 
ward seeds  is  repre- 
sented by  ferns.  You 
all  know  ferns.  Have 
you  ever  noticed  that 
sometimes  you  find  on 
the  under  surface  of 
fern-leaves  many  small 

brown  structures  ?  (See  Fig.  93.)  These  brown  structures 
are  made  up  of  many  little  cases,  and  each  one  of  the 
cases  contains  spores.  Ferns  do  not  have  seeds.  For  their 
spreading  abroad  over  the  land  they  have  to  depend  upon 
these  spores.  But  you  can  see  that  these  tiny  spores  have 
many  disadvantages  as  compared  with  seeds.  They  are 
not  protected  by  a  thick  wall  and -they  do  not  contain 


FIG.  93. — Maidenhair  fern  with  brown  spore 
cases  at  the  edges  of  the  leaves. 


THE  STORY  OF  SEEDS 


253 


FIG.  94. — Small  plants  produced 
by  the  fern  spores;  one  of  them 
is  producing  a  new  fern- pi  ant. 
—After  J.  M.  COULTER. 


much  stored-up  food.  Unless  they  fall  on  a  favorable  place 
very  soon  after  they  are  set  free  in  the  world,  the  spores 
of  common  ferns  soon  perish.  Seeds,  however,  may  live 
for  many  years. 

The  spores  of  ferns  do  not  produce  fern-plants.  They 
produce  a  little  flat  green  structure  that  looks  like  a  little 
liverwort  (see  Fig.  94).  This 
structure  produces  sperms  and 
eggs.  The  sperm  swims  to  the 
egg,  and  the  fertilized  egg  pro- 
duces the  fern-plant. 

Now  let  us  see  how  this  fourth 
step  (fern)  is  different  from  the 
third  step  (moss  and  liverwort). 
In  mosses  the  sperms  and  eggs 
are  produced  at  the  top  of  the 
little  baby  plants,  while  the  spores  are  produced  on  struc- 
tures that  have  no  leaves  (see  Fig.  92).  But  in  ferns  the 
spores  are  on  the  leaves,  while  the  sperms  and  eggs  are  pro- 
duced by  a  small  structure  that 
has  no  leaves.  These  two  things 
have  been  reversed.  You  see 
that  in  plant  life  there  are  two 
generations  —  the  spore-bearing 
generation  and  the  sperm-and- 
egg-bearing  generation.  In  the 
ferns  it  is  the  leafy  generation 
that  bears  spores,  and  this  is  also 
true  of  seed-plants. 

Now  we  are  ready  for  the  fifth 
step.  This  step  is  illustrated  by  a  very  graceful  plant  that 
is  common  in  greenhouses.  This  plant  is  a  distant  relation 


FIG.  95. — Diagram  of  a  flower.  5 
is  a  stamen;  0  is  an  ovary. 


254 


ELEMENTARY  SCIENCE 


of  the  common  ferns,  but  it  does  not  look  like  a  fern  at  all. 
Its  name  is  Selaginella  (club-moss).  This  plant  produces 
two  kinds  of  spores,  a  big  kind  and  a  little  kind.  The 
big  kind  produces  a  structure  that  bears  eggs  only.  The 
little  kind  produces  a  structure  that  bears  sperms  only. 
The  sperms  have  to  swim  about  to  find  the  eggs.  And 
still  there  is  nothing  that  looks  like  a 
seed! 

But  the  seed  appears  with  the  very  next 
step.  You  probably  know  that  flowers  pro- 
duce pollen.  Pollen  is  a  dust-like  sub- 
stance (commonly  yellow),  and  you  find  it 
at  the  ends  of  those  structures  inside  the 
flowers  that  are  called  stamens  (see  Fig.  95). 
Surely  you  have  heard  that  insects  (es- 
pecially bees)  carry  pollen  from  flower  to 
flower  as  they  go  about  searching  for  food. 
Now,  a  grain  of  pollen  is  a  spore.  But  we 
know  that  seed-plants  produce  two  kinds 
of  spores.  Where  is  the  other  kind  ? 

If  you  dissect  a  flower  you  will  nearly  al- 
ways find  at  the  very  center  of  it,  and  at 
the  bottom,  the  little  structure  that  is  go- 
ing to  become  the  fruit.  If  you  cut  open 
this  structure  you  will  find  inside  of  it  the  very  small 
structures  that  are  going  to  become  the  seeds.  These 
baby  seeds  are  called  ovules  and  the  structure  that  con- 
tains them  is  called  the  ovary  (see  Fig.  96).  It  is  inside 
the  ovules  that  the  big  spores  are  produced,  and  they 
never  escape.  It  is  this  that  gives  us  seeds.  The  big  spores 
produce  eggs.  These  eggs  are  hidden  away  inside  the 
ovules,  and  the  ovules  are  hidden  away  inside  the  ovary. 


FIG.  96.  —  Longi- 
tudinal section  of 
an  ovary,  show- 
ing ovule  and  pol- 
len tubes. — After 
J.  M.  COULTER. 


THE   STORY  OF  SEEDS 


255 


The  pollen  produces   the   sperms.    How  are  they  ever 
going  to  get  to  the  eggs? 

Look  at  the  picture.  Notice  the  slender,  stem-Hie  struc- 
ture that  rises  from  the  ovary.  You  can  find  this  in  nearly 
every  kind  of  flower.  At  the  very  top  of  this  structure 
there  is  a  rough  and  usually  sticky  surface.  This  is  the 
pollen-catcher.  Pollen  may  blow  on  it,  or  fall  on  it,  or 
be  rubbed  on  it,  or  be  carried  to  it  by  insects.  But,  how- 
ever it  gets  there,  when  it  gets 
there  it  stops,  and  it  begins 
to  grow.  It  sends  an  invis- 
ible tube  down  through  the 
stem-like  part,  and  then  into 
the  ovary,  and  then  into  the 
ovules  themselves.  When  the 
conditions  are  right,  there  is 
one  pollen- tube  for  each  ovule. 
These  invisible  pollen- tubes 
contain  the  sperms.  Now  you  can  see  how  the  fertili- 
zation of  seed-plants  occurs,  and  now  perhaps  you  can  see 
what  a  seed  really  is. 

The  fertilized  egg,  lying  there  inside  the  ovule,  begins 
to  grow  into  a  new  plant.  This  little  plant  that  is  formed 
inside  the  seed  is  called  the  embryo.  At  the  same  time, 
the  outer  part  of  the  ovule  hardens  and  forms  the  tough 
coat  of  the  seed  (see  Fig.  97).  Between  the  coat  and  the 
embryo,  food  is  stored  for  the  use  of  the  little  plant  when 
the  seed  begins  to  sprout.  If  you  study  botany  you  will 
learn  more  about  the  various  groups  of  plants  and  the  way 
they  are  related  to  each  other.  But,  whether  you  ever 
study  botany  or  not,  you  should  understand  something  of 
the  relationship  between  seed-plants  and  those  that  do  not 


FIG.  97. — Section  of  a  seed,  show- 
ing embryo  inside,  and  tough  coat 
outside. — After  J.  M.  COULTER. 


256  ELEMENTARY  SCIENCE 

produce  seeds.  You  should  understand  that  they  all  have 
the  same  general  problem  of  reproduction  to  solve.  The 
seed-plants  are  the  most  successful  plants  in  the  world, 
and  this  is  chiefly  because  the  seed  is  the  solution  of  the 
great  problem  of  spreading  out  and  multiplying. 

QUESTIONS 

1.  What  are  the  simplest  plants? 

2.  Did  the  first  plants  live  in  water  or  on  land? 

3.  How  do  the  simplest  plants  reproduce  ? 

4.  What  is  the  sex  process  of  reproduction? 

5.  How  do  land-plants  differ  from  water-plants  ? 

6.  What  land-plants  lived  before  there  were  any  seed-plants? 

7.  How  are  seeds  produced  ? 


CHAPTER  XXXIH 
PLANT  GROUPS 

In  the  last  chapter  you  have  had  the  story  of  the  seed. 
This  story  began  with  the  simplest  plants,  and  it  carried 
you  through  one  group  after  another  until  the  highest 
group  was  reached.  It  is  important  now  to  think  of  the 
plant  kingdom  as  a  whole,  so  that  you  will  know  the  great 
groups  when  you  see  them. 

Probably  most. people  think  of  plants  as  having  leaves, 
stems,  and  roots,  and  producing  flowers  and  fruits  and  seeds. 
This  is  true  of  our  most  conspicuous  plants,  and  of  the 
plants  that  are  most  useful  to  us.  But  you  ought  to  know 
that  the  plant  kingdom  includes  also  plants  without  seeds 
or  flowers;  and  even  plants  that  do  not  have  leaves  or 
stems  or  roots.  This  is  important  for  two  reasons:  first, 
you  may  see  these  more  simple  plants  anywhere  if  you 
know  about  them;  and  second,  it  is  from  these  simple 
plants  that  the  higher  ones  have  come. 

Those  who  study  plants  (botanists)  have  divided  up 
the  plant  kingdom  into  four  great  groups.  These  groups 
may  be  likened  to  the  four  stories  of  an  apartment-house, 
rising  one  above  the  other  until  the  highest  story  or  group 
is  reached.  You  should  know,  in  a  general  way,  the  kinds 
of  plants  that  live  in  these  different  stories. 

i.  The  First  Plants.  —  These  are  the  plants  that  live  in 
the  first  story.     They  are  called  the  first  plants  because 
once  they  were  the  only  kind  of  plants.     They  started  the 
25? 


258 


ELEMENTARY  SCIENCE 


plant  kingdom.  These  first  plants  are  of  two  general 
kinds,  and  you  have  seen  both  kinds.  One  kind  consists 
of  what  are  called  Alga.  You  have  seen 
green,  thready  growths  in  the  water, 
green  "scum"  in  ponds,  green  stains  on 
tree-trunks,  etc.  These  are  Algae,  our 
simplest  green  plants.  They  have  no 
leaves  or  stems  or  roots,  and  produce 
neither  flowers  nor  seeds.  If  you  have 
visited  the  seashore,  you  have  seen 
curious-looking  plants  thrown  up  on 
the  beach  by  the  waves.  Some  of 
them  are  brown  or  green  and  leathery; 

FIG.  98.— Seaweed.—     others  are  delicately  branching  and  red- 
After  J.  M.  COULTER. 

dish.    All  of  these  are  Algae  that  have 

learned  to  live  in  sea-water,  and  to  live  also  buffeted  about 
by  the  tides  and  storms.  Some  of  them  are  quite  large, 
and  they  are  commonly  called  "seaweeds" 
,  (see  Figs.  98  and  99).  In  many  places  along 
the  seacoast  these  seaweeds  are  thrown  up  on 
the  beach  in  great  numbers,  and  they  have 
been  found  to  be  very  useful  as  fertilizers. 
For  this  reason  they  are  carted  to  the  fields 
and  spread  over  them  like  manure. 

The  other  group  is  called  the  Fungi,  and 
they  differ  from  the  Algae  in  not  containing 
any  green  material.  For  this  reason  they 
are  unable  to  make  their  own  food,  and  must 
get  it  from  other  plants  or  from  animals. 
Some  of  them  attack  living  plants  or  animals  to  obtain  food, 
and  when  they  injure  them  they  produce  what  we  call  dis- 
eases. Many  of  our  diseases,  such  as  typhoid  fever,  diph- 


FIG.  99.  —  Sea- 
weed. —  After 
J.M.  COULTER. 


PLANT  GROUPS  259 

theria,  tuberculosis,  and  pneumonia,  are  caused  by  the  at- 
tacks of  certain  minute  Fungi  trying  to  obtain  their  food 
from  our  bodies.  In  doing  this,  they  either  destroy  some  of 
our  body  or  give  off  substances  that  poison  us.  Some  of 
these  Fungi  do  not  attack  living  bodies,  but  use  for  food 
dead  bodies  or  material  that  has  been  made  by  living 
bodies.  For  example,  you  may  have  seen  bread  that  has 
become  "mouldy."  This  means  that  one  of  these  Fungi 
has  attacked  the  bread  to  get  its  starch  (see  Fig.  46).  You 
know  about  yeast.  It  is  one  of  these  Fungi  that  attacks 
sugar,  and  in  so  doing  gives  off  a  gas  that  puffs  up  bread- 
dough  and  makes  it  " light." 

You  will  see  that  the  Algae  are  very  interesting,  because 
they  are  our  first  independent  plants,  independent  because 
they  are  green  and  can  make  their  own  food.  The  Fungi, 
however,  are  more  important  to  us  because  they  so  often 
produce  diseases  from  which  we  suffer,  and  also  diseases 
that  injure  our  valuable  animals  and  plants. 

2.  The  First  Land-Plants.  —  The  second  story  of  our 
plant  apartment-house  is  occupied  by  the  first  land-plants. 
The  Algae  live  in  water  or  in  very  moist  places.  They 
could  not  take  possession  of  the  land.  But  from  the  Algae 
the  first  land-plants  came.  You  have  seen  mosses,  which  are 
the  most  common  members  of  this  group.  They  are  found 
very  abundantly  in  shady  and  moist  places,  and  some  of  them 
are  able  to  live  in  very  exposed  places,  as  on  bare  rocks. 

The  plants  that  first  learned  to  live  on  land,  however, 
were  not  mosses,  but  the  relatives  of  mosses  known  as  liver- 
worts (see  Fig.  91).  You  will  find  them  in  moist  and 
shady  places,  sometimes  clinging  to  rocks  or  tree-trunks, 
but  sometimes  growing  flat  on  the  ground. 


260 


ELEMENTARY  SCIENCE 


Neither  liverworts  nor  mosses  have  flowers  or  seeds,  but 
they  made  great  progress  in  learning  to  live  on  land. 
The  danger  to  plants  in  living  on  land  was  that  they  would 
be  dried  out  and  killed  by  exposure  to  air.  If  a  fish  is 
taken  out  of  water  and  left  in  the  air,  it  cannot  continue 
to  live.  And  so  certain  Algae  gradually  learned  how  to 


FIG.  loo.— Ferns. 

protect   themselves   from   drying   out.     And   when   they 
learned,  they  were  no  longer  Algae,  but  became  liverworts. 

This  second  group  of  plants,  therefore,  took  a  great  step 
in  advance,  and  deserve  to  live  in  the  second  story.  They 
made  it  possible  for  plants  to  begin  to  occupy  the  land 
surface. 


3.  The  First  Woody  Plants.  —  These  plants  occupy  the 
third  story  of  our  apartment-building.     The  most  common 


PLANT  GROUPS 


261 


of  them  you  know  very  well,  for  they  are  the  ferns.  You 
know  that  ferns  develop  leaves  and  stems  and  roots,  and 
they  have  much  larger  bodies  than  do  the  mosses  and  liver- 
worts (see  Fig.  100).  In  fact,  you  know  that  some  of  the 
ferns  became  trees, 
which  you  may  have 
seen  in  greenhouses 
(see  Fig.  101). 

Why  were  ferns 
able  to  become  larger 
than  mosses?  The 
answer  is  that  they 
had  developed  some- 
thing new,  and  that 
was  woody  fibres. 
These  woody  fibres 
not  only  make  a 
more  rigid  body,  but 
their  chief  value 
came  from  the  fact 
that  they  carry 
water.  It  was  like 
putting  a  system  of 
water-pipes  into  the  plant.  By  means  of  these  water-pipes 
the  water  entering  through  the  roots  can  be  carried  rapidly 
and  directly  to  leaves  far  above  the  ground.  In  this  way 
plants  could  become  larger,  and  trees  became  possible. 

You  will  remember  that  you  have  seen  yellowish  or  black- 
ish dots  on  the  under-surface  of  many  fern-leaves.  These 
are  made  up  of  little  vessels  that  produce  small  bodies 
that  are  called  spores.  It  is  by  means  of  these  spores  that 
new  fern-plants  are  produced.  There  are  no  seeds  yet. 


FIG.  ioi.— Tree-ferns. 


262 


ELEMENTARY  SCIENCE 


Relatives  of  the  ferns  are  the  club-mosses,  which  perhaps 
you  have  seen  (see  Fig.  102).  Other  relatives  are  the  horse- 
tails, which  I  know  you  have  seen.  They  are  curious 

leafless  plants,  and 
when  they  are 
pointed  out  to  you, 
you  will  always  rec- 
ognize them  (see 
Fig.  103). 

4.  The  Seed-Plants. 
—  These  are  the 
plants  that  inhabit 
the  highest  story. 
They  are  woody,  but 
they  also  produce 
seeds.  The  story  of 
the  seed  is  given  in 
another  chapter,  and 
you  know  some- 
thing about  its  struc- 
ture. The  seed- 
plants  are  the  most 

successful  plants.  This  is  shown  by  the  fact  that  they  have 
taken  possession  of  so  much  of  the  land  that  they  seem  to 
cover  it,  where  plants  grow  at  all.  They  are  also  the 
plants  that  are  most  useful  to  us.  When  you  think  of  the 
many  ways  in  which  we  are  dependent  on  plants,  you  will 
see  that  it  is  the  seed-plants  that  supply  these  needs. 

You  should  know  the  two  groups  of  seed-plants.  You 
know  plants  that  belong  to  both  of  these  groups.  The  pines, 
hemlocks,  cedars,  etc.,  belong  to  one  group.  You  have 


FIG.  102. — Club-mosses. 


PLANT  GROUPS 


263 


probably  heard  them  called  "evergreens,"  because  their 
leaves  do  not  fall  off  in  the  autumn,  and  they  remain  green 
all  winter.  The  name  of  the  group  is  Gymnosperms,  which 
means  "naked  seeds."  The  seeds  are  not  enclosed  in  a 
case  or  fruit.  You  have  seen  a  pine-cone,  made  up  of 
hard,  overlapping  things  called  "scales."  If  you  break  off 


FIG.  103.— Horsetails. 

one  of  these  scales  you  will  find  one  or  two  naked  seeds 
fastened  to  its  upper  side  near  the  base.  Gymnosperms, 
therefore,  produce  seeds,  but  they  do  not  produce  flowers. 

The  other  group  of  seed-plants  is  very  much  larger,  and 
most  of  our  seed-plants  belong  to  it.  It  is  called  Anglo- 
sperms  because  the  seeds  are  produced  in  a  case  or  fruit. 
Angiosperms  means  ' '  seeds  in  a  case. ' '  You  know  that  you 
have  to  split  open  a  pea-pod  to  find  the  seeds;  and  that 
the  seeds  of  apples  are  in  the  "core." 

What  people  most  notice  about  Angiosperms,  however, 


264  ELEMENTARY  SCIENCE 

is  that  most  of  them  produce  flowers.    For  this  reason, 
Angiosperms  are  often  called  "flowering  plants." 

That  Angiosperms  are  much  more  successful  plants 
to-day  than  Gymnosperms  is  indicated  by  the  fact  that 
there  are  only  about  five  hundred  kinds  of  living  Gymno- 
sperms, while  there  are  at  least  one  hundred  and  forty 
thousand  kinds  of  living  Angiosperms. 

QUESTIONS 

1.  State  the  important   things  that  distinguish  the  four  great 

groups  of  plants. 

2.  What  are  the  differences  between  Algae  and  Fungi? 

3.  What  did  plants  have  to  learn  in  order  to  be  able  to  live  on  the 

land? 

4.  Which  one  of  the  classes  of  plants  has  been  the  most  successful  ? 

5.  What  is  the  difference  between  the  two  groups  of  seed-plants, 

and  are  both  groups  useful  to  us? 

6.  Of  what  use  is  a  flower  to  a  plant? 


CHAPTER  XXXIV 

RELATIONS  BETWEEN  PLANTS  AND  THEIR 
SURROUNDINGS 

Think  of  all  the  different  kinds  of  plants  that  you  might 
see  in  one  short  walk.  Think  of  the  microscopic  plants 
that  grow  in  water  as  compared  with  the  huge  trees  of  the 
forest.  Think  of  the  many  forms  of  mushrooms,  and  of 
the  strange  plants  of  the  tropics. 

Evolution.  —  We  wonder  why  there  is  all  this  great 
variety  of  forms  that  we  see  on  every  hand.  This  is  one 
of  the  great  problems  of  natural  science.  Scientists  for 
many  years  have  been  at  work  upon  it,  but  there  is  not 
yet  any  complete  answer.  You  know  that  we  do  not  be- 
lieve that  all  the  many  kinds  of  plants  and  animals  were 
separately  created.  We  do  believe  that  they  have  come 
from  common  ancestors.  This  great  process  is  called  evolu- 
tion, which  means  change.  It  means  the  production  of 
new  things  through  the  change  of  the  old  ones.  It  means 
that  in  ancient  times  as  well  as  to-day  no  two  forms  were 
alike  and  that  each  new  form  was  more  or  less  different 
from  its  parents.  You  can  see  that  with  this  sort  of  thing 
going  on  for  some  millions  of  years,  there  would  surely 
be  a  great  increase  in  the  number  of  kinds  of  plants  and 
animals. 

As  you  look  over  a  field  of  corn  you  are  quite  sure  to  see 
individuals  that  are  different.  Here  is  a  fine  plant,  nearly 
ten  feet  high,  and  bearing  two  good  ears.  Not  far  away  we 
265 


266  ELEMENTARY  SCIENCE 

find  one  much  shorter  and  with  only  one  good  ear.  Why 
this  difference  ? 

It  is  certain  that  the  difference  between  these  two  plants 
is  due  either  to  a  difference  that  they  inherited  from  their 
parents,  or  to  differences  in  their  surroundings  as  they 
grew,  or  to  both.  In  other  words,  all  living  things,  animals 
as  well  as  plants,  are  what  they  are  because  of  heredity 
and  environment.  Heredity  means  what  is  inherited  from 
parents;  environment  means  all  the  conditions  that  sur- 
round an  individual  after  birth. 

You  have  been  studying  the  environment.  You  know 
about  the  principal  physical  things  (light,  heat,  air,  etc.) 
that  affect  life.  Now  you  should  think  of  plant  life  as  a 
great  force  in  the  world  that  is  always  trying  to  express 
itself  (in  nutrition  and  reproduction)  in  the  presence  of  an 
environment  that  constantly  acts  upon  it  and  affects  it. 
This  has  been  going  on  for  such  a  very  long  time  that  we 
find  plants  usually  quite  well  adjusted  to  their  environ- 
ments. This  is  to  say,  the  shapes  and  structures  of  plants 
seem  to  be  a  good  deal  determined  by  their  surroundings. 
We  find  very  tough  and  hardy  weeds  growing  in  dry  places, 
while  soft  and  tender  plants  are  found  in  moist  and  shady 
places. 

Forests  are  the  highest  expression  of  the  plant  kingdom, 
and  the  most  interesting  study  of  plant  life  is  the  study  of 
the  woods  in  the  spring  when  the  leaves  are  coming  out  and 
the  early  spring  flowers  are  found  all  over  the  floor  of  the 
forest.  If  you  were  to  go  into  the  woods  some  fine  day  in 
late  April  or  early  May,  you  might  find  the  "spring  flora" 
at  its  best.  Then  you  could  stop  to  think  what  the  "  woods" 
really  means.  In  level  country  the  woods  usually  means 
great  age  of  the  topography.  The  trees  on  dry  bluffs  and 


PLANTS  AND   THEIR   SURROUNDINGS          267 

hillsides  are  never  so  large  as  those  that  grow  in  the  older 
valleys.  The  greatest  forests  in  the  world  are  in  the 
valleys  of  ancient,  slow-flowing  rivers.  The  greatest  of 
all  are  in  the  valley  of  the  Amazon. 

In  North  America  the  number  of  kinds  of  trees  is  much 
less  than  in  the  tropics.  There  are  about  twenty  kinds 
that  every  one  should  learn  to  know.  They  are  quite 
easy  to  recognize  if  you  will  practise  observing  them  very 
carefully  when  some  one  tells  you  the  names.  You  must 
practise  picking  out  the  characters  by  which  you  will  know 
them  the  next  time  you  see  them.  Oak,  maple,  ash,  elm, 
and  hickory  are  trees  you  all  should  know,  and  there  are 
several  kinds  of  each  one  of  these  that  you  will  soon  learn 
to  know  if  you  take  any  interest  in  plants. 

Now  going  back  to  the  woods  in  April.  Why  is  it  that 
so  many  beautiful  flowers  are  to  be  found  there  in  spring, 
while  in  summer  we  find  hardly  any  at  all?  Hepatica, 
violets,  spring-beauty,  bloodroot,  trillium,  Dutchman's- 
breeches  —  do  you  know  these  flowers  ?  In  April  and  May 
you  can  find  all  of  these  and  many  more.  But  in  July  and 
August  the  woods  are  bare  of  flowers.  Why? 

This  fact  shows  how  very  responsive  plants  may  be  to 
their  environment.  A  woods  is  a  sort  of  "plant  society." 
In  swamps,  thickets,  or  meadows  we  find  other  kinds  of 
plant  societies.  Every  society,  plant  or  human,  has  its 
more  and  its  less  prominent  members.  The  trees  are  the 
prominent,  controlling  members  of  forest  societies.  The 
other  plants  have  to  adjust  themselves  to  the  great,  success- 
ful trees.  The  trees  are  the  forms  that  have  made  sure  of 
the  light  and  of  the  moisture.  Their  leaves  are  high  in 
the  air  and  their  roots  deep  in  the  soil.  The  undergrowth 
must  do  the  best  it  can  with  the  light  that  filters  through. 


268  ELEMENTARY   SCIENCE 

Then  there  are  the  vines.  They  climb  up  the  trunks  of  the 
trees.  They  have  learned  how  to  take  advantage  of  the 
work  of  others.  Sometimes  in  the  forest  you  will  find  trees 
covered  with  vines,  weighted  down  and  killed  by  them. 

In  April  and  early  May  the  leaves  are  not  full  on  the 
trees.  Light  shines  through  to  the  forest  floor,  and  this 
is  the  season  that  the  little  plants  of  the  forest  have  made 
their  own.  In  a  few  weeks  they  do  nearly  all  their  work 
of  growth  and  of  reproduction,  and  then  rest  until  another 
season.  Nearly  all  the  early  spring  flowers  arise  from  bulbs 
or  other  kinds  of  thickened  underground  parts  in  which 
food  is  stored.  They  do  not  come  from  seeds.  They  shoot 
right  up,  and  often  open  their  flowers  even  before  the  leaves 
are  formed. 

So,  as  you  go  about  in  the  woods,  you  may  find  a  hundred 
ways  in  which  plants  are  closely  related  to  their  environ- 
ment. But  we  do  not  find  that  there  is  just  one  rule  of 
nature  that  explains  how  all  these  interesting  results  have 
been  achieved.  We  know  that  heredity  and  environment, 
working  together,  have  accomplished  the  results  that  we 
see,  but  we  cannot  say  just  what  is  due  to  one  and  what  to 
the  other.  Certainly  plants  differ  in  this  matter.  Some 
are  much  more  affected  by  their  surroundings  than  are 
others.  In  this  matter,  you  see,  plants  are  almost  pre- 
cisely like  people.  We  ourselves  are  the  result  of  our 
heredity  and  our  experience.  We  cannot  alter  our  heredity, 
but  we  must  make  our  experiences  as  desirable  as  possible. 


PLANTS  AND  THEIR  SURROUNDINGS          269 

QUESTIONS 

1.  What  is  evolution? 

2.  What  is  the  difference  between  heredity  and  environment? 

3.  Why  do  some  plants  grow  in  dry  places  and  others  in  moist 

places  ? 

4.  Where  are  the  greatest  forests? 

5.  Why  do  vines  climb  up  trees? 

6.  Why  do  we  see  certain  forest  flowers  growing  in  the  spring  but 

not  in  the  summer? 


CHAPTER  XXXV 
ANIMALS.    VERTEBRATES 

The  study  of  animals  is  called  zoology.  We  are  not  go- 
ing to  make  a  study  of  zoology  now.  That  will  come 
later.  But,  in  order  to  fill  out  your  picture  of  nature,  you 
need  a  definite  idea  of  the  animal  kingdom. 

Just  about  half  a  million  different  kinds  of  animals  have 
been  described  by  zoologists,  and  about  seven-tenths  of  these 
are  insects.  This  has  been  called  the  Age  of  Man  and  In- 
sects because  in  this  age  these  two  kinds  of  animals  have 
been  by  far  the  most  successful. 

Branches  of  the  Animal  Kingdom.  —  The  entire  ani- 
mal kingdom  is  divided  into  twelve  branches.  Of  these  there 
are  two  that  you  should  understand.  The  other  ten  may 
wait  until  you  study  zoology.  These  two  branches  are  the 
jointed-legged  animals,  and  the  vertebrates,  or  animals  with 
skeletons.  Of  the  former  there  are  several  classes,  three  of 
which  you  should  know.  A  crayfish  or  a  lobster  is  an  ex- 
ample of  one  of  these  classes,  the  class  called  Crustacea;  to 
another  spiders  belong;  while  a  third  is  made  up  of  that 
greatest  of  all  classes,  the  Insecta.  We  shall  study  them  in 
a  separate  chapter. 

The  Vertebrates.  —  Of  the  animals  with  skeletons  (Ver- 
tebrates), there  are  five  classes  you  should  remember: 
fishes,  amphibians,  reptiles,  birds,  and  mammals.  These 


VERTEBRATES 


271 


names  all  explain  themselves,  except  perhaps  amphibians, 
which  means  animals  that  can  live  either  in  water  or  on 
the  land.  Frogs  and  toads  belong  to  this  class.  Turtles 
and  alligators,  as  well  as  snakes  and  lizards,  belong  to 
the  reptiles.  Of  course  you  know  birds  when  you  see 
them,  but  it  may  surprise  you  to  know  that  birds  are  re- 
lated to  reptiles;  that  is,  the  first  bird  was  a  sort  of  winged 
reptile. 

The  last  and  highest  class  of  the  vertebrates  are  the 
mammals,  to  which  we  ourselves  belong.  Nearly  all  the 
domestic  animals  are  also  mammals,  and  many  others 
that  are  wild  furnish  us  with  food  or  clothing.  The  name 
mammals  refers  to  the  mammary  glands  of  the  female  that 
furnish  milk  for  the  young.  The  fur-bearing  animals  are 
all  mammals.  Fishes,  birds,  amphibians,  and  most  reptiles 
lay  eggs,  but  mammals  give  birth  to  their  young  alive. 

Every  boy  and  girl  should  try  to  get  clearly  and  per- 
manently in  mind  the  way  in  which  mammals  are  classified 
into  different  groups.  This  will  help  you  greatly  in  organ- 
izing your  knowledge.  There  is  no  more  fascinating  book 
than  Hornaday's  "American  Natural  History"  in  which 
you  can  read  of  the  habits  of  all  the  important  American 
vertebrates. 

The  mammals  are  divided  into  thirteen  orders,  four  of 
which  it  is  quite  important  to  remember.  First  come  the 
Primates,  which  means  "first  order."  To  this  belong  the 
apes,  the  monkeys,  and  man.  Then  come  the  carnivorous 
(flesh-eating)  animals.  To  this  order  (Ferce)  most  of  our 
interesting  wild  animals  belong.  This  order  is  composed 
of  five  families:  cats,  dogs,  bears,  martens,  and  raccoons. 
Wild  members  of  the  cat  family  in  America  are  the  puma 
and  the  lynx,  while  wolves  and  foxes  belong  to  the  dog 


272  ELEMENTARY  SCIENCE 

family.  The  marten  family  includes  our  small  fur-bearers: 
otter,  mink,  weasel,  wolverene,  skunk,  badger.  You  should 
learn  to  know  all  these  animals  by  studying  them  in  a 
museum  or  zoological  garden  if  you  have  a  chance. 

The  third  order  of  mammals  you  should  remember  are 
the  Rodents,  or  gnawers.  Here  belong  rats  and  mice, 
and  also  squirrels,  rabbits,  and  gophers.  The  fourth  im- 
portant order  is  composed  of  the  hoofed  animals — Un- 
gulates. Cattle,  deer,  sheep,  swine,  and  horses  are  the  most 
important  members  of  this  great  order. 

Among  mammals  belonging  to  the  smaller  orders  are 
seals,  moles,  bats,  whales,  and  opossums.  Now  as  you 
stop  to  think  of  all  these  forms  you  realize  what  a  great 
and  successful  class  the  mammals  are,  and  how  they  have 
spread  all  over  the  world. 

Think  then  of  the  fishes  in  the  water,  the  birds  in  the 
air,  amphibians  on  both  land  and  water,  and  you  realize 
that  these  animals  with  skeletons  have  come  to  occupy 
the  earth  as  far  as  large  animals  can  go.  All  other  great 
groups  of  animals  are  small  in  stature  as  compared  with  the 
vertebrates. 

Now,  having  in  mind  all  this  wonderful  population  of 
vertebrates,  let  us  think  how  they  are  built  to  do  their  work 
in  the  world.  They  must  all  move  actively  about  in  search 
of  their  food.  Their  skeletons  form  a  solid  framework  to 
which  muscles  are  attached,  and  the  movements  are  all  ac- 
complished by  muscular  contractions  acting  on  the  bones. 

But  what  is  it  that  moves  the  bones?  And  what  acts 
on  the  food  and  oxygen  that  enter  the  body?  How  are 
they  changed  so  that  they  can  be  used  ?  Perhaps  you  can 
answer  these  questions  as  to  your  own  body,  but  do  all 
these  other  vertebrates  have  lungs,  hearts,  nerves,  blood- 


VERTEBRATES  273 

vessels,  stomachs,  and  intestines  the  same  as  you?  They 
all  do  except  the  fishes,  which  have  gills  in  place  of  lungs. 

As  to  reproduction,  you  learned  in  your  study  of  plants 
that  the  sex  method  is  used  in  some  of  the  very  lowest 
plants  and  in  all  of  the  high  ones.  But  all  plants  also  use 
other  methods.  In  the  higher  animals,  however,  there  is 
only  the  sex  method.  Sperms  are  produced  by  the  males, 
and  eggs  by  the  females,  and  these  must  be  united  in  the 
mating  of  the  sexes.  The  fertilized  egg  grows  directly 
into  a  new  individual,  and  there  is  no  resting-stage  that 
corresponds  with  the  resting-stage  you  have  observed  in 
seeds. 

The  great  bulk  of  the  body  of  all  these  animals,  whether 
it  be  a  fish  or  a  frog,  a  bird  or  a  mammal,  is  devoted  to  the 
work  of  securing  food  and  making  use  of  the  food  after  it 
is  obtained.  In  that  respect  they  are  just  the  same  as 
plant-bodies,  although  they  are  so  differently  organized  into 
definite  organs. 

The  intelligence  of  animals  is  related  to  the  size  of  their 
brains.  All  the  vertebrates  have  bony  skulls  which  pro- 
tect the  brains  within  them.  As  we  progress  from  fishes 
toward  man,  we  find  the  brain  growing  larger  and  larger, 
until  in  man  we  find  developed  that  wonderful  organ  that 
makes  all  the  difference  between  man  and  beast.  It  is 
only  in  brain-power  that  we  excel  the  lower  animals.  They 
can  excel  us  in  all  the  other  organs.  Their  hearts,  muscles, 
stomachs,  lungs,  and  sense-organs  may  all  be  better  than 
our  own.  It  is  only  in  our  more  highly  developed  nervous 
tissues  that  we  excel  them.  We  can  think,  reason,  and 
imagine.  And  this  makes  us  responsible  for  our  own  acts. 
The  lower  animals  are  safely  guided  by  their  instincts. 
We  are  not.  All  our  life  long  we  must  seek  self-control. 


274  ELEMENTARY  SCIENCE 

There  is  no  insanity  among  animals,  but  insanity  appears 
quickly  among  human  beings  where  self-control  is  lost. 
We  alone  of  all  living  things  are  responsible  for  our  own 
acts.  It  is  the  price  we  pay  for  being  at  the  top  of  the 
scale.  We  can  be  the  happiest  of  all  creatures.  We  can 
also  be  the  most  miserable.  Suicide  and  crime  are  un- 
known except  among  more  highly  civilized  human  beings, 
and  here  we  would  not  have  them  except  for  failure  of  the 
brain  to  do  its  work  properly;  failure  to  assert  itself  and 
be  the  real  master  of  the  body. 

Another  thing  that  increases  as  brain  size  increases  is 
the  length  of  time  that  parents  care  for  the  young.  Take 
the  fish.  Here  there  is  no  care  for  the  young  at  all.  There 
is  not  even  mating.  The  male  discharges  sperms  in  the 
water  just  as  the  female  discharges  eggs.  Fertilization 
takes  place  entirely  outside  the  bodies  of  the  parents, 
and  is  very  uncertain.  Eggs  and  sperms  are  produced  by 
millions.  If  they  were  not,  the  race  of  fishes  would  soon 
disappear. 

When  we  come  to  snakes,  however,  we  find  real  care 
of  the  young;  while  birds  are  placed  next  below  mammals 
for  no  better  reason  than  the  attention  they  give  to  the 
rearing  of  then:  young.  It  is  with  human  beings,  of  course, 
that  this  matter  reaches  its  extreme  form.  Human  parents 
are  responsible  for  the  protection  and  care  of  their  children 
for  many  years.  Of  course  this  means  that  much  fewer 
young  are  produced.  So  the  investment  of  time  and 
money  and  devotion  to  each  one  is  greatly  increased,  and 
young  people  represent  a  very  large  investment  by  their 
parents  and  by  society  in  general.  It  is  truly  a  terrible 
thing  not  to  take  this  investment  seriously  and  responsibly. 


VERTEBRATES  275 

QUESTIONS 

1.  What  is  the  largest  class  of  animals? 

2.  What  are  the  five  classes  of  vertebrates?    Give  examples  of 

each. 

3.  How  do  mammals  differ  from  all  other  animals? 

4.  Name  some  of  the  classes  of  mammals,  and  give  examples. 

5.  Is  a  whale  a  fish? 

6.  What  methods  of  reproduction  are  there  in  the  higher  animals? 

7.  In  what  respect  does  man  excel  the  lower  animals? 

8.  How  do  different  kinds  of  animals  differ  in  the  care  of  their 

young? 


CHAPTER  XXXVI 

INSECTS 

Man's  two  greatest  enemies  are  himself  and  insects. 
The  best  friends  he  has  among  insects  are  the  cannibal 
insects,  those  that  serve  him  by  devouring  other  injurious 
forms.  Bees  yield  us  honey,  and  silk  comes  from  the 
cocoons  of  a  certain  moth,  but  how  gladly  we  should  give 
up  silk  and  honey  if  thereby  we  could  escape  the  harm 
that  other  insects  do  to  our  crops  and  to  our  very  lives! 
The  common  house-fly  is  the  most  dangerous  animal  in 
the  world;  it  carries  typhoid  fever.  And  there  is  a  kind 
of  mosquito  that  spreads  malaria  from  patient  to  patient. 
Altogether,  we  seem  to  be  a  good  deal  at  the  mercy  of 
insects.  Flies  and  mosquitoes  we  can  drive  out  if  we  take 
the  trouble  to  do  so,  but  there  are  many  insect  enemies 
of  our  important  plants  that  we  do  not  know  how  to  con- 
trol. The  gypsy  and  the  brown-tail  moths  are  very  injuri- 
ous to  shade-trees,  and  millions  of  dollars  have  been  spent 
in  Eastern  United  States  in  trying  to  control  them,  and  yet 
their  ravages  continue.  Our  annual  financial  loss  due  to 
insects  is  estimated  as  follows: 

Cereals $300,000,000 

Hay  and  forage , 66,500,000 

Cotton 85,000,000 

Tobacco 10,000,000 

Truck  crops 60,000,000 

Sugar 9,500,000 

Fruit , 60,000,000 

Farm  products 11,000,000 

Miscellaneous  crops 10,000,000 

276 


INSECTS  277 

Animal  products $300,000,000 

Natural  forests 100,000,000 

Products  in  storage 200,000,000 


$1,212,000,000 

Twelve  hundred  millions  in  a  year  that  might  be  saved 
if  we  knew  just  how  to  fight  these  tiny  enemies  instead  of 
fighting  them  in  the  dark !  It  is  like  shooting  into  a  forest 
to  hit  enemies  we  cannot  see. 

The  study  of  insects  is  called  entomology,  and  in  the 
employ  of  the  state  and  national  governments  there  are 
many  economic  entomologists  who  devote  their  whole  time 
to  finding  out  how  to  fight  these  very  injurious  insects. 
As  has  been  suggested  before,  one  of  the  very  best  ways  is 
to  find  another  insect  that  is  an  enemy  of  the  injurious 
one.  Once  such  an  insect  is  found,  it  is  carefully  bred 
until  a  great  hostile  army  of  them  has  been  produced.  Then 
these  are  set  free  to  destroy  the  kind  that  is  hurting  the 
crops,  or  the  fruit-trees,  or  whatever  it  may  be.  In  Cali- 
fornia a  certain  kind  of  beetle  (Vedalia)  is  kept  in  large 
colonies  by  the  State  Horticultural  Commissioner.  There 
is  another  kind  of  insect  (cottony-cushion  scale)  that  is 
very  bad  for  the  oranges.  Whenever  there  is  danger  of  an 
epidemic  of  the  scale-insect,  colonies  of  the  beetle  are  sent 
to  the  "firing-line,"  and  they  promptly  exterminate  the 
cottony  scale.  This  is  a  case  in  which  almost  perfect 
control  has  been  worked  out,  but  it  took  many  years  to 
do  so,  and  the  protecting  insect  was  finally  located  hi  far- 
away Australia.  Kellogg  and  Doane's  "Economic  Zo- 
ology" is  a  very  good  book  in  which  to  read  about  the  dif- 
ferent kinds  of  injurious  insects  and  the  ways  in  which  they 
are  combated. 


278 


ELEMENTARY   SCIENCE 


Quite  apart  from  their  importance  in  dollars  and  cents, 
insects  are  very  interesting  creatures  to  study.  They  have 
wonderful  "life  histories,"  and  very  much  is  yet  to  be 
found  out  about  their  behavior.  You  already  know  the 
life  history  of  moths  and  butterflies,  and  you  know  of  the 
wonderful  behavior  of  ants  and  of  bees. 

Some  insects  appear  to  be  the  most  intelligent  of  all 
animals  lower  than  man,  and  yet  we  cannot  be  sure  that 
they  are  reasoning  creatures. 
You  see  the  world  is  a  very 
different  place  to  an  insect 
from  what  it  is  to  us.  It 
breathes  differently,  sees  dif- 
ferently, smells  differently,  and 
in  practically  every  way  has 
a  plan  of  organization  wholly 
different  from  our  own. 

The  two  great  ideas  about 
insects  that  you  should  have 
axe  (i)  an  idea  about  how 
they  are  constructed  and  how 
they  live,  and  (2)  an  idea  of 
the  classification  of  insects;  that  is,  an  idea  of  the  six  great 
groups  and  of  the  important  insects  belonging  to  each. 

All  insects  have  their  hard  part  outside  and  their  soft 
part  inside.  They  have  no  skeleton,  as  vertebrates  have. 
There  are  three  distinct  parts  of  the  body:  head,  thorax, 
and  abdomen.  To  the  thorax  three  pairs  of  legs  are 
attached.  The  wings  are  also  attached  to  the  thorax. 
On  the  head  are  the  compound  eyes,  composed  of  a  great 
number  of  small  eyes  lying  very  close  together.  Just  in 
front  of  the_eyes  arise  the  antenna,  which  are  very  impor- 


Ab. 


FIG.  104. — Diagram  of  the  body  of 
an  insect;  H,  head;  T,  thorax; 
Ab,  abdomen;  An,  antenna. 


INSECTS 


279 


tant  organs.    They  are  used  for  feeling  and  for  smelling, 
and,  in  some  insects,  for  hearing  (see  Fig.  104). 

On  the  lower  side  of  the  head  we  find  the  peculiar  mouth. 
Insects  do  not  have  lips,  tongue,  and  teeth  in  the  ordinary 
sense.  They  have  what  are  called  mouth  parts,  and  these 
six  mouth  parts  are  variously  arranged  in  different  insects. 
In  a  grasshopper  they  are  arranged  for  biting.  In  a  bug 
they  are  arranged  for  piercing 
and  slicking.  In  a  fly  they  are 
arranged  for  lapping  (see  Fig. 


One  of  the  most  surprising 
things  about  an  insect  is  that  it 
breathes  with  its  abdomen.  That  is 
to  say,  along  the  side  of  the  ab- 
domen are  found  little  openings 
called  spiracles.  These  open  into 
tubes  called  trachea,  which  branch 
and  run  to  all  parts  of  the  body, 
even  out  into  the  wings. 

Now  think  of  the  activity  of 
an  insect  as  compared  with  your 
own.  How  many  times  the  length 
of  its  own  body  can  a  grasshopper  jump?  How  many 
times  can  you?  It  is  perfectly  evident  that  insects  far 
excel  us  in  muscular  activity  in  proportion  to  their  size. 
A  man  would  have  no  trouble  jumping  over  the  Wool- 
worth  Building  if  he  could  jump  like  a  flea.  The  breath- 
ing-plan of  insects  has  a  good  deal  to  do  with  their  activity. 
It  permits  oxygen  to  get  very  quickly  to  all  parts  of  the 
body.  In  other  words,  insects  do  not  get  "out  of  breath" 
the  way  we  do. 


FIG.  105. — Drawing  to  show  the 
complicated  mouth  parts  of 
an  insect. 


280  ELEMENTARY  SCIENCE 

The  blood  of  insects  does  not  go  through  closed  vessels, 
as  in  man.  It  fills  that  part  of  the  body-cavity  not  occu- 
pied by  other  organs.  There  is  a  true  circulation,  however, 
the  heart  being  located  just  under  the  middle  of  the  back. 
The  digestive  system  is  well  developed,  while  there  is  a 
central  nervous  chain  that  passes  along  the  lower  side  of 
the  body. 

Of  the  six  great  orders  of  insects,  you  should  understand 
that  two  of  them  have  incomplete  metamorphosis,  while 
four  of  them  have  complete  metamorphosis.  This  long  word, 
metamorphosis,  names  a  very  interesting  fact  of  insect  Life. 
You  probably  know  that  a  caterpillar  changes  into  a  moth 
or  butterfly.  It  is  this  complete  changing  from  one  form 
into  another  in  the  life  of  an  individual  that  is  called 
metamorphosis.  In  a  complete  metamorphosis  there  are 
three  stages.  The  egg  hatches  into  a  worm-like  stage 
called  larva.  A  caterpillar  is  a  kind  of  larva.  It  is  in  the 
larval  stage  that  insects  do  most  of  then"  eating.  It  is  in 
this  stage  that  they  are  so  harmful  to  crops  and  to  foliage. 
Larvae  eat  almost  constantly  throughout  their  lives.  After 
they  have  stuffed  themselves  and  reached  full  growth, 
they  pass  into  a  resting-stage,  the  pupa  stage.  During 
this  stage  they  undergo  a  quite  complete  change  of  form. 
When  they  emerge  from  it  they  usually  have  wings.  They 
are  now  in  the  reproductive  or  egg-laying  stage  (see  Fig. 
106). 

Early  fall  is  the  best  time  of  year  in  which  to  study 
insects.  In  early  September  they  are  at  the  height  of 
their  activity.  You  can  always  find  then  representatives 
of  the  six  great  orders. 

First,  the  grasshoppers.  They  belong  to  an  order  called 
Orthoptera,  which  means  straight  wings.  Their  metamor- 


INSECTS  281 

phosis  is  incomplete.  The  eggs  are  laid  in  the  ground,  and 
they  grow  directly  into  young  grasshoppers.  Crickets 
also  belong  to  this  order.  The  members  of  it  all  have 
strong  jumping  legs. 


FIG.  106.— Metamorphosis  of  the  insect. 


Then  the  bugs.  It  is  common  to  call  all  insects  that 
are  not  well  known  "bugs."  But,  properly  used,  this 
is  a  scientific  term  that  names  just  one  order  of  insects, 
the  Eemiptera.  This  word  means  half-wings,  and  refers 
to  the  fact  that  in  many  bugs  the  wings  are  very  small. 
Some  of  them  have  no  wings  at  all.  Most  of  them  have 
flat  bodies  that  permit  them  to  live  in  cracks  and  crevices. 
In  September  you  are  quite  sure  to  find  a  good  many  bugs 


282  ELEMENTARY  SCIENCE 

trying  to  get  into  houses  for  winter  quarters.  June  "bugs " 
and  potato  "bugs"  are  not  real  bugs,  but  are  beetles. 
Some  bugs  have  a  complete  metamorphosis,  but  most  of 
them  do  not. 

The  beetles  are  very  numerous,  but  most  of  them  stay 
hidden.  You  find  them  abundantly  in  rotting  wood  or 
under  the  ground.  The  so-called  "  pinching-bug "  is  one 
of  the  commonest  forms  of  beetles.  The  outer  wings  of 
beetles  are  hard  and  are  not  used  in  flying.  Hence  the 
name  of  the  order,  Coleoptera,  which  means  sheath-wings. 

Then  there  is  the  great  group  of  flies  which  includes 
mosquitoes.  These,  you  know,  are  the  great  disease-car- 
riers, and  in  that  way  do  more  harm  to  man  than  all  other 
kinds  of  insects.  They  are  called  Diptera,  which  means 
two  wings.  You  know  that  insects  regularly  have  two  pairs 
of  wings,  but  in  flies  and  mosquitoes  there  are  only  two 
wings.  The  second  pair  of  wings  (hind  pair)  found  in 
other  insects  has  been  changed  in  the  flies,  being  repre- 
sented by  two  small  rods  called  "poisers,"  because  they 
are  believed  to  help  the  insect  to  keep  its  balance  when 
flying. 

Then  there  are  the  butterflies  and  moths,  which  every 
one  knows.  The  name  of  their  order  is  Lepidoplera,  which 
means  scale-wings,  referring  to  the  fact  that  their  wings 
and  bodies  are  covered  with  scales  that  are  usually  colored. 
It  is  very  common  to  see  the  brightly  colored  butterflies 
and  moths  flitting  about  flowers  and  sucking  out  the 
nectar. 

The  sixth  order  contains  the  bees  and  wasps  and  ants, 
and  is  named  Hymenoptera,  which  means  membrane-wings, 
referring  to  the  thin,  papery  wings.  It  is  much  the  largest 
order,  and  contains  very  important  and  interesting  insects. 


INSECTS 


283 


I 


i 


Bug. 


Locust. 


Beetle. 


Jutterfly. 


"*-* 


Fly. 
FIG.  107. — The  various  orders  of  insects. 


Spider. 


You  know  something  of  the  habits  of  bees  and  wasps  in 
building  homes  and  storing  food.  The  ants  are  everywhere 
and  seem  to  be  peculiarly  intelligent,  living  in  communities 
that  are  very  wonderful  in  their  organization  (see  Fig.  107). 


284  ELEMENTARY  SCIENCE 

QUESTIONS 

1.  How  are  insects  useful,  and  how  are  they  harmful? 

2.  Describe  the  body  of  an  insect. 

3.  How  does  an  insect  breathe? 

4.  How  does  the  blood  system  of  a  man  differ  from  that  of  an 

insect  ? 

5.  Describe  the  metamorphosis  of  an  insect. 

6.  What  are  the  different  classes  of  insects?    Give  some  examples 

of  each. 


INDEX 


Abdomen,  of  insects,  279 
Aeration,  93 
Air,  i,  166,  167 

—  a  mixture  of  gases,  43 

—  composition  of,  151,  156 

—  entering  water,  42 

—  expansion  of,  180,  181 

—  movements  of,  19 
Air-pressure  (see  atmospheric  pres- 
sure) 

Algae,  249,  258 
Aneroid  barometer,  58 
Animals,  i,  270-275 

—  jointed-legged,  270 

—  vertebrates,  270 
Antarctic  Circle,  176 
Antennae,  278 
Anticyclone,  193 

Archimedes  (287-212  B.  C.)  prin- 
ciple, 63,  64 
Arctic  Circle,  176 
Artesian  wells,  33,  52 
Astronomy,  172 
Atmosphere,  3 

—  convection  currents  of,  167 

—  defined,  147 

—  density  of,  148,  149 

—  humidity  of,  143 

—  in  indoor  life,  199 

—  relation  to  heat,  150,  167 

—  relation  to  sun,  162 

—  weather,  a  state  of,  178 
Atmospheric  pressure,  55-62 

—  at  mountain  tops,  59 

—  at  sea-level,  59 

—  cyclones,  192 

—  high  and  low,  180 

—  measuring,  58 

—  relation  to  water  vapor,  184 

Bacteria,  101-104 
Barometer 

—  aneroid,  58,  59 

—  high  and  low  pressures,  180 

—  mercurial,  59,  61 


Caloric,  124 

Calorimeter,  133,  134 

Calory,  132,  143 

Cancer,  Tropic  of,  175,  176 

Candle-power,  228 

Capricorn,  Tropic  of,  175, 176 

Carbohydrates,  230,  234,  238 

Carbon,  159 

Carbon  dioxide,  43,  153,  154,  156, 

158 

Cells,  244 

Centigrade  thermometer,  130 
Chlorophyll,  238 
Cistern,  46 
Clay,  52,  84,  94,  96 
Clouds,  4 

—  cirrus,  197 

—  cumulo-nimbus,  197 

—  stratus,  197 
Coal,  3,  50 
Cold,  135-140 

—  freezing,  136 
Color,  216-224 

—  rainbow,  220 

—  solar  spectrum,  220 

—  waves,  220 
Combustion,  209-215 

—  burning,  209 

—  energy  released  by,  214 

—  products  of,  213 

—  rusting,  209 
Condensation,  13,  16,  18,  143 
Contraction,  12,  89,  129,  141,  144 
Convection,  145,  167,  180 
Convection  currents,  167,  180 
Crustacea,  270 

Cycle,  of  life,  233 

—  nutritive,  236 
Cyclones,  192 

—  anticyclone,  193 

Day,  169,  174 

Degree,  of  latitude  and  longitude, 

175 
Deposition,  25 


285 


286 


INDEX 


Dew,  1 8 

Diffusion  of  gases,  44 

—  of  light,  223 
Digestion,  9 
Doldrum,  186 
Drainage,  83 

Earth,  as  planet,  170 

—  axes  and  poles,  172 

—  crust,  52 

—  depth,  47 

—  inclination  of  axes,  172 

—  inside,  47 

—  movements  of,  169-177 

—  orbit,  171 

—  revolution  around  sun,  170, 171 
Electrolysis,  158 

Energy,  117,  164 

—  combustion,  214 

—  conservation  of,  137,  138 

—  forms,  117 

—  heat,  a  form  of,  216 

—  kinetic,  118,  119,  142 

—  light,  a  form  of,  216 

—  potential,  118,  119,  142 
Entomology,  277 
Environment,  266 
Equator,  185 
Equinoxes,  173 

—  storms,  1 74 
Erosion,  25,  28 
Ether,  217 
Evaporation,  5,  9,  10,  16,  143 

—  from  leaves  (transpiration),  35 
Evolution,  265 

—  environment,  266 

—  heredity,  266 

Expansion,  12,  89, 129, 141, 143,  144 

Fahrenheit    (1686-1736)   thermom- 
eter, 130 
Fats,  234 
Feldspar,  94 
Fertilization,  251 
Fertilizers,  107 
Filters,  68 
Fishes,  270 
Fog,  18 
Food,  5,  232-240 

—  carbohydrates,  234 

—  fats,  234 

—  inorganic,  233 


Food,  nitrogenous,  234 

—  non-nitrogenous,  235 

—  nutritive  cycle,  236 

—  organic,  233 

—  oxidation  of,  238 

—  proteins,  235 
Freezing,  136 
Frost,  1 8 

Fulcrum,  112,  114 
Fungi,  105,  106,  258 

—  mushrooms,  106 

—  puffballs,  106 

—  toadstools,  106 
Fusion,  heat  of,  143 

Galileo  (1564-1642),  124 
Gas,  3,  53 
Glaciers,  35,  36 

—  glacial  till,  38 

Gravel,  33,  52 

Gravitation,  119,  120,  149 

Gravity,  16,  21,  74,  75  (law  of  gravi- 
tation, 22) 

Great  Basin,  191 
Ground-water,  31 

Hail,  19 

Heat,  i,  4,  9,  13,  49,  137-140,  145 

—  and  atmosphere,  1 50 

—  calorimeter,  128 

—  conduction,  137,  166 

—  convection,  145,  167,  180 

—  day  and  night,  169 

—  form  of  energy,  216 

—  measurement,  128 

—  radiation,  165 

—  solar,  172 

—  thermometer,  124, 128, 130, 131 

—  variation,  141,  142,  180 

—  vibration,  217 
Heating,  200-208 

—  fireplace,  200 

—  furnace,  202 

—  stove,  202 
Heredity,  266 
Houses,  190-200 
Humidity,  17,  143 
Hurricane,  185 
Hydration,  91,  92,  160 
Hydraulic  press,  65 
Hydrogen,  155,  157,  158,  159 


INDEX 


Ice,  9,  138,  142,  145 

—  glaciers,  35,  36 

—  sheet,  37 

Inorganic  (see  substances) 
Insecta,  270 

Insects,  276,  283 
Inventions,  2 
Iron  oxide,  210 
Irrigation,  80,  81 
Isobar,  184 
Isotherm,  184 

Kinetic  theory,  125 

Lakes,  32,  46 

Larva,  280 

Latitudes,  174 

Lava,  49 

Leaves,  244 

Lenses,  226 

Lever,  112,  114 

Light,  i,  4,  165,  216-224 

—  absorption,  221 

—  diffusion,  223 

—  form  of  energy,  216 

—  intensity,  229 

—  rainbow,  220,  225 

—  rays,  222 

—  refraction,  222,  226 

—  reflection,  222,  227,  228 

—  solar,  172 

—  spectrum,  220 

—  translucent,  224 

—  transmission,  224 

—  transparent,  224 

—  waves,  217,  219 
Liquid,  3 
Longitude,  274 

Mammals,  270 

—  orders  of,  primates,  272 

—  rodents,  272 

—  ungulates,  272 
Man,  273 

Matter,  9  (see  substances) 
Mercury,  59,  60,  61,  62,  128 
Meridians,  175 
Metamorphosis,  280 
Microscope,  227 
Minute,  175 

Molecular  motion,  139, 140, 141, 163 
Molecules,  12,  13,  155-160 
Monsoons,  187 


Moraines,  38 

Motion,  218  (see  kinetic  theory) 

Mountains,  worn  down,  30 

—  rainfall,  191 

—  range,  51 

Natural  science,  i 

Nature,  i,  2 

Newton,  Sir  Isaac  (1642-1727),  120 

Night,  169,  174 

—  variation,  1 74 
Nitrates,  160 
Nitrogen,  43,  151,  152,  156 

—  importance  to  plants,  103, 104 
Nutrition,  248 

Nutritive  cycle,  236 

Oil,  53 

Orbit  (of  earth),  171 
Organic  (see  substances) 
Oxidation,  91,  160,  161,  238 
Oxygen,  43, 151-160,  209 

Parallels,  174 
Pascal  (1623-62),  61 
Photography,  227,  228 
Photosynthesis,  229,  230,  238 
Phlogiston  theory,  210 
Piston,  75,  76 
Planets,  170 

—  axes,  1 70 

—  movements,  1 70 

—  revolution,  1 70 

—  rotation,  1 70 
Plants,  i 

—  algae,  249,  258 

—  annuals,  247 

—  effect  of  sunlight,  230 

—  fungi,  258 

—  perennials,  247 

—  pollen,  254 

—  reproduction,  248,  249 

—  stamens,  254 
Polar  regions,  169 
Pollen,  254 

Pressure,  low  and  high,  21 
Proteins,  235 
Protoplasm,  160,  232,  236 
Pulley,  113 
Pump,  55,  56,  57,  61,  66 

Quartz,  94 


INDEX 


Radiation,  164,  165,  205 
Rain,  4,  14 

—  thunder-storms,  195 

—  uses  of,  189 
Rainbow,  220,  225 
Rainfall,  189,  100 

—  effect  of  mountains,  191 

—  in  Mississippi  Valley,  194 

—  in  United  States,  31 
Reclamation  Service,  82 
Reflection  of  light,  222 
Refraction,  222 
Refrigeration,  179 
Revolution  of  earth,  172 
Rivers,  24,  46 

—  deposition,  24 

—  erosion,  24 
Rock,  50,  88 

—  disintegration,  92 

—  formation,  49,  51 

—  granite,  51 

—  layers,  32,  50,  51 

—  limestone,  49,  51,  52 

—  sandstone,  51,  52 

—  sedimentary,  51 

—  shale,  51 
Roots,  243 

Rotation  of  earth,  172,  185 
Rust,  210 

Sand,  52,  84,  94 

—  porous,  33 
Sap,  41 
Saturation,  13 
Seasons,  169-177 

—  causes,  160-177 

—  changes  of,  169 
Sediment,  41 
Seeds,  247-256 

—  algae,  249 

—  fertilization,  251 

—  reproduction,  248,  249 
Siphon,  58 

Smoke,  213,  214 
Snow,  189,  190 

*°—  auuVii,  96 

—  eolian,  97,  98 

—  fertility,  101,  107 

—  food-supply,  107 

—  formation,  87 

—  grains,  84 


Soil,  layers,  32 

—  nature  of,  09 
Solar  system,  169-177 
Solid,  3 

Solution,  40-45 

Soot,  213 

Sound,  216-224 

Spectrum,  220,  225 

Spores,  252 

Springs,  32,  33,  46,  52 

Stalactites,  47,  48,  52 

Stamens,  254 

Stand-pipe,  68 

Stars,  1 20,  149 

Steam,  9,  10 

Steam-power,  70-78 

Strata,  33,  50,  52 

Subsoil,  52 

Substances,  inorganic,  233,  242 

—  organic,  233,  242 
Sun,  147 

—  centre  of  solar  system,  170 

—  effect  on  atmosphere,  162 

—  revolution  of  earth  around,  1 70, 

171 

—  rising  and  setting,  171 

Telescope,  227 

Temperature,  128  (see  "Heat  and 
Cold,"  135-Uo) 

—  of  day  and  night,  169 

—  of  land  and  water,  139 
Thermometer,  128,  129,  130 

—  centigrade,  130 

—  Fahrenheit,  130 
Thermostat,  208 
Thunder-storms,  195 
Topography,  35 
Tornadoes,  197 
Torricelli's  experiment,  60,  61 
Trade-winds,  182 
Transpiration,  35 
Turbine,  72,  73 

Vacuum,  163,  164 

Vapor,  10 

Ventilation,  199,  200,  206,  207 

Vertebrates,  270-272 

Volcanoes,  48,  49 

Water,  8,  16,  46,  67,  89 

—  air  enters,  42 


INDEX  289 

Water,  erosion,  25-27  Weather,  atmospheric  movements, 

—  extent,  8  179 

—  filtered,  44  —  forecasts,  178 

—  ground- water,  31  —  maps,  182 

—  hard,  44  Weather  Bureau,  182 

—  irrigation,  80  Weight,  120-121 

—  material  dissolved,  44  Wells,  32,  46,  52,  53 

—  refraction  of  light  in,  223  —  artesian,  33,  52 

—  soft,  44  —  oil,  53 

—  supply,  85  Windlass,  55 

—  suspension  in,  44  Winds,  3,  178-188 
Water-meters,  68  —  basic  directions,  186 
Water-power,  70,  71,  72,  74,  77  "~  convection  currents,  182 
Water-pressure,  63,  64,  65,  66  =  "JjJ  '« 
Water-table,  32  _  movements  of,  179 
Water-works,  66,  67  _  pianetary,  187 
Waves,  color,  220  —  prevailing,  186 

—  length,  218  —  trade,  182 

—  light,  219 

—  sound,  218  Zenith,  172 

—  water,  218  Zones,  174-177 
Weather,  178-188  Zoology,  270 


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